How DC-Activated Railroad Crossing Detection Works

Picture of access road to Lincoln Air Base
Figure 1--Access road to Lincoln Air Base.

Introduction and Outline

This is one of a series of pages describing how the how the circuitry works at an active, full size, "real" railroad crossing in North America.  The pages are intended to be read in the order given on the home page.  This page is specifically about DC-activated detection circuits and other closely related circuits, especially directional stick relays.  Links to pages discussing other types of crossing circuits may be found on this page or on the Signal Department home page.  As a condition for accessing the information on this and certain other pages all readers must first accept the website's "Disclaimer and Terms of Use" which relates critical safety information regarding the content and defines how and when the content is permitted by its authors to be applied or shared.  

The outline for this page is as follows:

  1. Introduction and Outline
  2. General Information on Detection Circuits
  3. DC-Activated Crossings
    1. Crossings Using No Direction Sensing
    2. Crossings Using Interlocking Relays
    3. Crossings Using Directional Stick Relays
  4. Credits

Included is an actual railroad circuit print showing the complete wiring of the location pictured below, which used stick relays at the time the print was current.

Before we begin, it is convenient to review Figure 2 again, which illustrates the principle and function of the master relay.

Diagram illustrating function of the master relay.
Figure 2--The function of the master relay.

Most crossing installations, unless they are entirely solid state, have a normally energized master relay as shown in Figure 2.  The relay is often designated XR and was discussed in greater detail on the first page.  The key concept is that when a train is approaching the detection circuits "drop" the master relay to activate the signals.

The circuits on either side of the master relay in Figure 2 may be simple or complex.  They may be relays themselves or they may be solid state.  In discussing them, the XR forms a good point of reference.  On this page, the circuits discussed will be some of those that could exist on the left side of Figure 2, up to and including the master relay.

General Information on Detection Circuits

The section of track leading up to the crossing on each side is called an approach.  The short portion of track that extends through the road is called the island.  Each crossing, therefore, usually has one island between two approaches.  Each approach has a definite length--say, 2000 feet for example--while the island may reach to just a few feet outside the edges of the pavement or be a little longer.  Generally, the detection system drops the master relay (Figure 2) any time a train is on the island regardless of other conditions such as movement, speed, or direction.  The behavior of the control circuits for trains on the approaches varies depending on the type of control circuit.  In the explanations that follow, the approach that the train enters on will be called the "leading" approach, while the approach that a train exits on will be called the "trailing" approach.

Virtually all active railroad crossings in North America detect trains by sending electrical current through the rails.  This is called a track circuit.  The circuitry connected to the track responds in some way to electrical changes caused by passing trains.  Although some creative men have proposed alternative detection methods, the track circuits work so well compared to any known alternative that any such alternatives can still be considered very experimental in nature.  However, track circuits have been invented to work on various electrical principles, and many different designs and improvements that work on the same principles have been implemented.

Sometimes the sense of the term track circuit, or simply circuit for short, refers not strictly to the circuit itself but more broadly to include the portion of track through which it operates.  Likewise, the words approach and island may refer to either to the respective track sections or to the electrical a circuits that operate in them.  As an example, signal department employees may be heard to say that a train is "on the circuit," which means that the train is on the section of track so as to be detected by the associated circuit.  It may also be said that a crossing has "1,100-foot DC approaches," for example, meaning that there are physical features in the track which are intentionally placed there to allow the detection circuit to be effective for precisely that length of track.

DC-Activated Crossings

The oldest railroad crossings use one or more DC track circuits for detection.  In a DC track circuit, the range of detection is sharply defined by insulated rail joints at each end of a given length of track.  A track battery is connected across the rails at one end of the track section, and a DC track relay connected across the rails at the other end.  When the track section is not occupied by a train, current passes through the rails to the relay, causing the relay armature to be picked up.  But when a train is on the circuit, the metal wheels and axles of the train create a shunt on the track; that is, they create an easier path from one rail to the other, shunting the current away from the relay coil, causing it to drop out.  Whether or not a train is actually present, the current path includes a current limiting resistor at the battery end to prevent trains from creating a "dead short" on the battery.  See Figure 3.

Simplified DC Track Circuit.
Figure 3--Simplified DC Track Circuit.

Because these are DC track circuits, the crossings that use them are said to be DC activated crossings--a commonly used term used to refer to crossings with "old" circuits.  But being a DC-activated crossing technically depends only on the track circuit portion even though the other parts of the crossing circuitry may be DC as well.

For reasons that should become evident later on this page, most crossings that use this technology have two or more adjacent track circuits.  The contacts of each track relay are then placed in control of the XR, as shown in Figure 4.  The remaining wiring then becomes a solution to the problem of making the warning stop, for example, after the rear of the train has cleared the crossing.

Multiple Track Relays in Control of the XR.
Figure 4--Multiple Track Relays in Control of the XR.>

DC-Activated Crossing Using No Direction Sensing

In the primitive days of crossing control, the signals were activated whenever a train was occupying a given length of track spanning the crossing.  The signals apparently continued to operate after the rear of the train cleared the crossing until it cleared the trailing approach as well, some thousand or so feet away.  Although this could have been accomplished with a single, very long DC track circuit covering both approaches, the actual design may have still used two adjacent track circuits--each including one approach--to allow the track relays of both circuits to be located in the case beside the road, with the track batteries at the far ends of the approaches, as shown in Figure 5.  This design would most closely accomplish the purpose in a manner consistent with Figure 2 and Figure 4.

Sample wiring for Constant Warning Distance with No Direction Sensing.
Figure 5--Sample wiring for Constant Warning Distance with No Direction Sensing.>

In some cases, there might have been only one track circuit, but in these cases the wires from the relay end of the rails would need to come back to the relay case track relay over lengthy line wires to provide closed-circuit monitoring of the entire length of track.  Without more than one track relay in the control of the XR, some railroads likely combined the functions of the track relay and XR into a single relay that might have had some other designation.  Although this consolidation does not fit as neatly into Figure 2 or Figure 4, it probably would have been preferred by some railroads at one time as the effort involved in maintaining aerial pole line wires was considered little or no object compared to the logical simplicity of the circuit and the chance to save space in tiny, pole-mounted relay cases. (See Figure 6.)

Sample wiring for Constant Warning Distance with No Direction Sensing.
Figure 6--Sample wiring for constant warning distance with no direction sensing, using only one track circuit.>

As a compromise between Figure 5 and Figure 6, it also would have been possible to have a total of two relays where the track relay from one approach controlled the signals as in Figure 6 while contacts of the track relay from the other approach simply broke the track wires to the first relay.

Although the Figure 5 and Figure 6 show a familiar flashing light signal in the lower, right-hand corner for ease of understanding, the kind of control circuits they represent are very very old and would more likely be used with simpler types of signals, as with a just a bell and a steadily illuminated "DANGER" sign.

In the early days, the intent of a crossing signal seems to have been to warn of an unseen train on an approach rather than of a train occupying on the island, which the motorist could presumably see for himself.  The warning given was thus more of an advisory statement, supplementary to the motorists' powers of observation and judgement.  If the warning continued after the train passed, it would probably not have been frowned upon to cross the tracks while the signal operated.  The persistence of the signal to operate was mostly not perceived as a problem except due to the noise generated by the crossing bell.  Regulations that came into being around 1929 required the cessation of warning after the train passed, so the train could no longer ring out on a long trailing approach.

A Single Track Circuit Crossing.  This crossing /could/ use a single DC track circuit like the one above.
Figure 7--A Single Track Circuit Crossing.  This crossing could use a single DC track circuit like the one above.

There are still some crossings today that do not have direction sensing circuitry, although they are used with slow speed track and do not have long approach sections.  Typically, they have a single track circuit that extends just far enough out from the road on either side to allow the train to activate the signals and stop just short of the road.  Special signs and rules require the crew to bring the train to a complete stop and then wait a certain number of seconds while the signals operate before proceeding over the crossing.  

DC-Activated Crossings Using an Interlocking Relay

Since a warning should only be given from the approach on one side of the road, the side from which the train enters, most DC-activated crossings had a way of selecting only that approach and blocking the warning from the other approach based on which track circuit was occupied first.  The ability to determine this is referred to as direction sensing.  The first direction sensing solution in widespread use was that of the "crossing relay," usually referred to more descriptively as the interlocking relay due to its design.  It could make train detection effective for only the leading approach and lock out the warning on the trailing approach, allowing the signals to clear after the rear of the train passed.  This worked well for complete, through train movements regardless of the direction of the train.

An interlocking relay basically works by having essentially two relays in the same case, side-by-side.  In the simplest application, coils from each side of the relay are treated as track relay coils, being energized or de-energized depending on the occupancy of their respective east and west approach track circuits at the crossing.  Both armatures may operate independently with the exception that only the first side to be de-energized may have its armature drop fully and its contacts change state.  The other armature, if dropped while the first one is still down, can fall only into a "half-drop" state where its contacts do not change state.  The second-dropped armature will not be allowed to reach full drop unless both sides are again energized simultaneously, it which case it may fully drop if its coils are the first de-energized by a subsequent train.  The unique behavior of this relay is made possible by a mechanical interlocking system internal to the relay.  

Figure 8 shows the simplest interlocking relay application.  The X between the two sides of the relay denotes the interlocking relationship between what otherwise could be mistaken for two separate relays, and they are not wires.  This example shows the signals controlled through the back contacts of the interlocking relay.

Diagram showing interlocking relay in operation.
Figure 8--Crossing Relay in Operation.

When used as simply as above, an interlocking relay can represent be the totality of the crossing detection and logic, and it is sometimes designated as the master relay itself.  Yet there are can be additional complexity by adding additional track circuits, by isolating it from the track circuits, by adding a stand-alone master relay to control signals, and more.  An interlocking relay cannot be used to control normally energized signal circuits through its front contacts except where special allowances are made.  Because interlocking relays are considered quite antiquated but yet have many interesting design possibilities, a separate page is dedicated to covering interlocking relay designs in greater detail.

DC-Activated Crossings Using Directional Stick Relays

Stick relay designs are an alternative to an interlocking relays and are usually considered more modern while having the disadvantage that they require more space.  Typically, stick relay designs begin with three adjacent track circuits: one for the island and one for the approach on each side.  These three track circuits each have their own plain track relay, and through the contacts of these relays the control of the XR may be broken to activate the signals.  The interlocking relay discussed earlier is replaced by two stick relays and some fancier wiring.  The stick relays are also relatively plain relays by design, which are named stick relays because of how they behave when wired to do their job.  Thus the detection equipment is less specialized than in the interlocking relay design.

Unlike interlocking relays, crossing stick relays are de-energized with no trains present.  They are controlled by some contacts of the track relays and also some contacts of themselves.  When a train goes through, one or the other of the two stick relays will stick up through a pickup circuit based on which track circuit is occupied first, thereby achieving a kind of direction sensing.  In concept, the stick relay that first sticks up remains energized or "stuck up" through one or more holding circuits until the train is gone and all three track circuits are again unoccupied.  The pickup circuit for the opposing (i.e., the other) stick relay is broken by a back contact of the first stick relay so that it is not energized at any time during the train movement.  The circuits are symmetrical so that a train going through in the other direction would energize the other stick relay only.

One of the simplest designs using stick relays is shown in Figure 9 below.  The bottom of the figure shows the chain of track relay contacts in control of the XR after the manner of Figure 4.  It can be seen that a front contact of each stick relay bypasses the contact of a track relay.  This bypassing is ultimately the only purpose of a stick relay.  The warning can then stop after a train has cleared the crossing even though the track circuit on the trailing approach may still be occupied by that train.

Stick relays in operation with pickup upon reaching the approach.
Figure 9--Stick relays in operation with pickup upon reaching the approach.

Stick relays are usually slow release relays, as implied by the dark band drawn at the bottom of the coil symbol.  This can be achieved either by using relays that have this feature built in or by connecting condensers, resistors, or rectifiers across the coils.  If the slow release feature were not used, momentary glitches such as those caused by poor shunting could cause the stick relay to drop out and, under some conditions, allow the opposing stick relay to become falsely energized.

In the analysis of stick circuits, there is room for confusion between the letters E and W used for East and West in the designations.  A westbound train enters first on the east approach, but it is on the west approach that the warning is locked out.  So which letter is used in the designations of each relay?  In the author's survey, it seems that this problem is often avoided with track relays, which are given numbers or letters such as 1TR, 2TR, 3TR; or ATR, BTR, CTR.  With stick relays, on the other hand, the current of popularity seems to go with the bound convention, where WSR, for example, is rendered as the "Westbound Stick Relay," that is energized for a westbound train (which, of course, bypasses the track relay contact in control of the XR for the approach on the west side of the crossing).  Nevertheless, the author has found a stick relay designated WBSR on a written circuit, which was wired to energize for an eastbound train.  Therefore, let the reader beware: the student of signal circuits must be prepared to discern the meanings of the designations on a case-by-case basis.  Some schemes will also insert the letter X for CROSSing somewhere in the designations.

It is important to understand that some designs have the stick relay sticking up immediately when the train enters the leading approach, while other designs do not allow the stick relay to energize until the train has both occupied the leading approach and also reached the island.  If the stick relay sticks up when the train first enters the approach, as in the above circuit, this can allow for fault conditions where the warning does not occur until the train reaches the island, and does not afford many safety advantages over the interlocking relay.  If the system is designed so that the stick relay does not energize until the train reaches the crossing, however, it reduces the likeliness that the warning will be locked out in the face of an approaching train.  Thus stick relays have the potential to be much safer than interlocking relays depending on how they are wired.  Another sample wiring scheme, in which the stick relays are not allowed to energize until the train reaches the island, is shown below.

Stick relays in operation with pickup upon reaching the island.
Figure 10--Stick relays in operation with pickup upon reaching the island.

With reference to the bottom portions of the above circuit examples, the order of the track relay contacts in control of the XR can vary.  The author has found circuits drawn so that the contact of the island track relay appears before, after, and between the two approach track relay contacts in control of the XR in different drawings.  However, it most often appears first in the chain as shown.  This is because being first allows the heel carrying battery to serve both the front contact in the control of the XR and the back contact in the stick relay holding circuit.  In other words, what is drawn as two sets of contacts may actually be one.  On an actual print, the contacts are all numbered, and the number may be the same in both places.

The above examples show a locomotive running light (i.e., without cars) through the crossing.  Although trains have cars more often than not, stick relay designs must correctly handle the case where the train is shorter than the island.  Stick relays in both of the above-given wiring schemes have a holding circuit that includes a back contact of the island relay to cover that situation.  Yet, in some schemes this holding circuit does not exist.  In these schemes, the holding of a stick relay when a train is totally contained within the island may be achieved indirectly through the use of a kind of trap circuit.  Supply to the approach track relay coils are broken by front contacts of the track relays themselves and of the island relay in parallel.  When an approach track relay drops, it cuts of the supply to its own coil so that it cannot be re-energized unless the island relay is also energized.  So, the approach track relay will remain down until the train has cleared not only the approach, but the island also.  With the approach track relay thus controlled by the island track relay, it is not necessary to have a stick holding circuit for the island alone.  

Figure 11 shows such a design, which is a variation on the design in Figure 10.  Note that although the circuits immediately controlling the stick relays are similar in both designs, the holding circuit involving a back contact of the island track relay in Figure 10 is absent from Figure 11 where this function is performed indirectly by the approach trap circuits.  A similar modification could be made on the design in Figure 9 as well.

Stick relays in operation with trap circuits in approach track circuits.
Figure 11--Stick relays in operation with trap circuits in approach track circuits.

Another advantage to this design is that it might theoretically reduce the probability of "hiccups" due to poor shunting conditions as a short train crosses from the approach into the island.  A possible disadvantage, though, would be that logic becomes mixed in with the detection circuits in such a way that in normal operation the approach track relays no longer provide absolute information about their respective track sections, which could make troubleshooting confusing and more difficult.  In the author's view, it seems that approach trap circuit designs were not as widely used as designs that did not have them.  Its resemblance to some advanced interlocking relay designs suggests that it might have been a carryover from the interlocking relay era that was perhaps dropped in later stick relay designs.

Some designs would have the holding circuits as well as the pickup circuit broken by the back contact of the opposing stick relay.  There are potentially many variations to stick circuit design.  And for every given design, it could be found drawn differently so as to be mistaken for a different circuit.  The ability to retain basic principles and learn anew from a quick inspection of circuit prints are valuable on the railroad.

Picture of former C.R.I.&P. grade crossing.
Figure 12--Former C.R.I.&P. (Rock Island) grade crossing on the Abilene & Smoky Valley Railroad in August, 2005.  The signals have since been replaced by passive crossbucks.

Picture of open signal case.
Figure 13--Open signal case at the above crossing.  Relays are on the top shelf, and batteries are on the floor.  Power supplies and other pieces of equipment are located on intermediate shelves.

Close-up of relay shelf within case.
Figure 14--Close-up showing top shelf, the edge of which bears stenciled designations of the relay.  The stick relays are called "XES" and "XWS" in this railroad's scheme.  Note extra signal lenses and reflectors stored on second shelf from the top.  (See bottom of page for photo source info.)

Entire Crossing Print From the Stick Circuit Crossing Above
File Type File Size File Name
Web Page
(Use this link to view the entire print without downloading the file separately.)
780 KB xing-stickprint
(Use this link to download the entire print in reduced size with landscape orientation for easy printing on a single page.  Requires PDF reading software.)
803 KB xingprint2.pdf
GIF Image
(Use this link to download the raw image of entire crossing print.)
749 KB xingprint2.gif

As with interlocking relay designs, all stick relay designs still use the order of track circuit shunting to determine the direction of train movement.  Consequently, the system cannot tell the difference between a train that has stopped on the trailing approach and a fault that has occurred in the associated track circuit.  If the trailing approach track circuit should fail as a westbound train is leaving, for example, there could be no warning from the west approach when an eastbound train enters.  This is because the westbound stick relay would still be energized from the westbound train.  To help alleviate this problem, a further safety improvement can be made by the introduction of stick cutout modules.  A stick cutout module contains a timer that interrupts the supply to the stick relay coil after several minutes (say, 12 minutes) of the train's clearing the island.  This does not fix the track circuit problem on the trailing approach, but it unmasks it by allowing the signals to activate again from the trailing approach after a short time.  In normal operation, then, a train must clear the area before time expires or the signals will re-activate until it does.

If a train were to stop on the trailing approach and then reverse toward the crossing before time expires on the stick cutout, there will still be no warning before the train reaches the island.  In practice, one would hope that the train crew is aware of this and that they account for it in one of three ways: 1) stop and let one of the crew protect the crossing as a flagman, 2) creep up in the train slowly enough that the island circuit starts detecting before the road is blocked, or 3) observe that there are no cars near enough to be warned and go about their merry way.  (The third solution is the author's favorite.)  An automatic warning in this situation is something that only the constant warning time or motion detector circuits can provide as discussed on a later page.


  1. Helpful influence provided by E. E. Ford, BNSF signal maintainer, retired.
  2. Lots of other helpful conversation provided by Jerry S.
  3. Pictures of signal case photographed by author with permission of Abilene & Smoky Valley RR.

Was there anything on this page that you found particularly interesting, helpful, useful, educational, or enjoyable?  Please contact the author and let him know!

Contact Info

How Railroad Crossing Signals Work
How Interlocking Relay Railroad Crossing Designs Work
How Motion Detector and Predictor Railroad Crossing Detection Works

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