How Motion Detector and Predictor Railroad Crossing Detection Works

Picture of access road to Lincoln Air Base
Figure 1--A crossing that uses modern detection circuitry.

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 the form of train detection associated with motion detectors, predictors, and other closely related circuits.  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. Constant Warning Time Using Predictors
  3. Wrap Circuits
  4. Motion Detectors
  5. Credits

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.

Constant Warning Time Using Predictors

The relay circuits like the ones described on previous pages can determine only if track circuits are occupied or not, and which ones are occupied first, etc.  They cannot tell at what speed the train is approaching the crossing.  They also cannot in what direction the train is going without assuming complete through movements.  Some special relay circuits were used to get better information, but the information was still very much approximate.  Relay circuits are essentially constant warning distance circuits.

It is not ideal to have to have a constant warning distance that a warning will be given within, but a constant warning time so that the signals will be activated no sooner or later than they need to be for all train speeds and directions.  The warning should also stop if the train stops or backs away before reaching the crossing.

As a solution, some relatively modern circuitry uses the equipment configuration shown in Figure 3.  The circuitry involved is too complex to show or describe in complete detail.  Moreover, the author's understanding is limited (so beware).  Hence, only the basic facts are given.

Print showing railroad crossing track circuits for constant warning time.
Figure 3--Railroad crossing track circuits for constant warning time.

Transceivers (combination transmitter/receivers) located inside the Box put low frequency a.c. signals on the track.  Some significant distance from the crossing, shunts are connected across the rails, which may simply lie on the track between them.  The shunts are tuned shunts that behave approximately as short circuits at the frequency of the electrical signals placed on the track.  As such, when no trains are present, practically all the current from the transceiver goes through the shunt that is tuned to it the same way that in a d.c. circuit practically all the current would go through plain wires placed across on the track.

The transceiver measures the voltage on the track and the current through it on a time base, from which the system can calculate the impedance and phase angle of the electrical load as seen from the crossing.

As the train enters the space between the transceiver and the shunt, the impedance drops.  And it continues to drop more and more as the train gets closer to the crossing.  The voltage decreases and the current increases (unless the current is regulated).  In this way, the transceiver knows not only that the train is within the distance to the shunt, but how close to the crossing it is.

This relatively precise position information can be differentiated in a mathematical sense to determine the train's speed, and this in turn can be used to calculate when the train will arrive.  The apparatus that performs these calculations is called a crossing predictor.  An output from the electronic equipment feeds the coil of the master relay, and when it decides the time is right, it turns off this output to drop the relay and thus start warning the automobile traffic a preprogrammed number of seconds before the projected arrival time.  The warning can be canceled if the train stops or reverses, and start again with movement toward the crossing.

HXP-3 Predictor card rack and interface.  The cards on the top and bottom rows should be identical.  One row is the normal card rack, and the other row is the backup card rack.
Figure 4--HXP-3 Predictor card rack and interface.  The cards on the top and bottom rows should be identical.  One row is the normal card rack, and the other row is the backup card rack.

Having a train on the island should make the signals active regardless of speed or direction.  This is called presence detection.  Theoretically, it might be possible to determine the presence of the island region from the same position sensing circuits described above, but there may be another special a.c. track circuit between points B and C to monitor it.

The a.c. track circuits for a predictor crossing do not usually need insulated joints to delimit the island or the approach sections of track.  Where turnouts exist in the crossing area, however, an insulated rail joint is still needed in the diverging closure rail.  An insulated switch rod is needed for the same reason.

HXP-3 Predictor terminals.  These are the AAR terminals that connect the backplane to the outside world.
Figure 5--HXP-3 Predictor terminals.  These are the AAR terminals that connect the backplane to the outside world.

Wayside signals may also exist on the line, whose train detection areas overlap with, but are usually not the same as, the detection area for the crossing predictor.  The two types of equipment therefore must each have their own track circuits and yet share the rails by using different frequencies in conjunction with filters where necessary to block each other's electrical impulses.  Furthermore, where insulated joints are placed for the wayside signals' benefit between the predictor (the Box) and the shunts, filters may be necessary to pass the transceiver's signals around them.  (See Figure 6.)

Diagram Showing Filters Around Insulated Joints
Figure 6--Insulated joints for wayside signals being filtered around for crossing circuits.

A predictor or motion detector system has many self sanity checks to make sure everything from the track circuit to the predictor itself is OK.  For instance, the system knows what phase angle is appropriate for a given impedance.  It responds to many faults by activating the signals until they are corrected.  Consequently, because it is somewhat sensitive to faults, there is a possibility that crossing gates will be unnecessarily blocking a crossing for hours until the signal maintainer arrives and finds the problem.  It is thus desirable to have a redundant system that allows automatic fail-over to the backup equipment when a problem occurs.

Wrap Circuits

The automatic fail-over feature mentioned above is available with new equipment today, but historically it was either unavailable or cost prohibitive.  A more accessible but less integrated and less desirable solution is the use of wrap circuits.

Wrap circuits use track circuits that are analogous to the simpler d.c. track circuits used with interlocking or stick relays discussed on another page, which can tell if a train is anywhere in the track circuit but not how close it is.  When a crossing is operating from the wrap circuit, its performance at best reverts to something like constant warning distance circuitry.  The warning given due to a train will be longer than usual, but at least it will not persist indefinitely.

A simple manner in which the predictor and the wrap circuits share control the master relay is shown in Figure 7.  In this "wired-OR" configuration, either the wrap branch or the predictor branch of the circuit may energize the XR and clear the signals.  Conversely, both must be de-energized to allow the XR to drop and a warning to be given.

Diagram Illustrating Wrap Circuit Control Principle
Figure 7--The principle involving control of the XR by either the wrap circuits or the predictor/motion detector circuits.

When there are no faults, the wrap branch stops feeding the XR whenever a train is in the track circuits that are part of the wrap, but the functioning predictor keeps it energized through its branch because it's not necessary to activate the signals quite yet.  When the predictor feels that the time is right, it de-energizes its branch of the XR also, allowing the XR to drop out and the warning to be given.  In the case where the predictor has failed, the predictor control branch is permanently de-energized, so the XR drops as soon as the wrap circuit branch de-energizes.

Unlike some of the earliest detection circuits, wrap circuits usually do not even have the logic to stop the warning after the train has cleared the crossing until it also clears the wrap detection area on the opposite side.  The warning starts when the train is in the area and stops when it leaves: it's as simple as that, insofar as the predictor is nonfunctional.  The motoring public is fortunate if the railroad line in question is dark territory (meaning no wayside signals for the trains themselves), because the track circuits in the wrap need be no longer than the length of the crossing approach on each side (say, 1,000 to 2,000 feet).

However, if the line is signaled, the situation is even worse because the wrap circuits do not use track circuits dedicated to the purpose, but instead rely on information from the track circuits used by the wayside signals, which are grossly longer than the approach length of the crossing and don't begin and end where convenient.  (See Figure 8.)

Diagram Illustrating Scope of Wrap Detection.
Figure 8--The scope of detection for a wrap circuit in signaled territory.

Figure 8 shows two highway grade crossings on a railroad line having wayside signals.  The location of track shunts (like those in Figure 3) are marked as "SHUNT," which represent the outer limits of the predictor's train detection on each side of that particular crossing.  Within the shunts, a fully functional predictor may decide that the train is coming soon enough to activate the warning.  However, since the wrap circuits depend on the track circuits of the wayside signals, we see that a failed predictor at the Spruce Street crossing would force the signals to activate as long as a train is somewhere in track section 4T--regardless of movement, speed, direction, or the fact that the rear of the train may have already cleared the crossing.  Another crossing at Maple Boulevard is within the track section 8T.  However, 8T does not cover the full length of the west approach to the crossing, and 10T must be included.  A failed predictor at Maple Boulevard would force the signals to activate as long as a train is somewhere within either of the track sections 8T or 10T.  The wayside track circuits might each be, say, 1-3/4 miles long.

From the illustration, it can be seen that a wrap circuit area wraps around a normal zone of detection, which is probably how they get named "wrap" circuits.  It can also be seen that wrap circuits aren't very kind to the public, but they call attention to the predictor failure problem without blocking the crossing as much as it would be without any kind of backup at all.  Railroads could use special, shorter track circuits that are designed just for the wrap, in spite of the wayside signals, as well as including some directional stick relays to cut the warning out after the train had cleared the crossing, but the author is not aware that this has ever been widely done.  Apparently the fact that the wrap circuit represents a failure mode is enough to keep it from being at all fancy.

Motion Detectors

When an approaching train stops close to the crossing but yet outside the island section, it is appropriate for the warning to stop.  However, when the train starts again, it is necessary to activate the warning immediately in order to have enough time for gates to be lowered and for people to get out of the way.  (Sometimes, when the train starts very close to the crossing, the gates are not even fully lowered by the time the train gets there.)  In this case, it is not useful to calculate the arrival time of the train but only to have the signals respond immediately upon train movement.  This is the behavior of a motion detector or detector for short; it activates the signals when the train is moving toward the crossing and de-activates them when it has stopped on the approach or is moving away.

A good predictor would in the same case calculate that the signals needed to activate immediately, so the motion detector has no theoretical advantage over a predictor that the author is aware of.  Probably the only reason why motion detectors are used instead of predictors is that a predictor is more complicated and hence expensive, since a predictor must calculate when the train will arrive.  When a train enters the approach, the activation is immediate regardless of train speed.  This has the disadvantage that for complete, through movements the behavior is the same as that for a constant warning distance circuitry.  The motion detector has great advantages over constant warning distance with stick relays, though, in that it can stop the warning if the train stops or reverses on the leading approach; it also has the safety advantage that it can restart the warning if the train reverses toward the crossing on the trailing approach.  For these reasons, a motion detector may be used at a location where most crossing activations are the result of slow speed switching moves where stopping, starting, and reversing near the crossing are likely.

The motion detector uses a setup almost identical to that of a crossing predictor, using track shunts and wrap circuits.  Both are boxes about size of an automotive battery charger and full of electronics.  Because the main difference between them is behavioral, some things about a predictor installation can be learned from looking at motion detector installation and vice versa.

PMD-2 motion detector card rack (with cover open) and terminal board.  The cards on the left and right are virtually identical.  The cards on the left are for normal operation, and the cards on the right are for standby operation.
Figure 9--PMD-2 motion detector card rack (with cover open) and terminal board.  The cards on the left and right are similar.  The cards on the left are for normal operation, and the cards on the right are for standby operation.

Below is a simplified diagram of a motion detector setup, which is inspired by prints for an actual motion detector installation.  The key point to notice is that the outputs of the box are connected to relays MDR and MD-ILR, which tend to control the master relay, XR.  MD-ILR drops out whenever a train is present on the island, even if no motion is detected.  The MDR on the other hand, theoretically drops out whenever motion toward the crossing is detected inside the distance to the shunts.  However, the motion detection output will also drop the relay under any of several other conditions as well--most notably whenever island presence is detected.  The transmitter and receiver are connected on opposite sides of the island.  The seemingly redundant track wires shown in Figure 10 are check wires that would in effect activate the signals if any of the track wires should break.  Modern apparatus has the transmitter and receiver as two different cards in the same box rather than essentially comprising two separate boxes, and a normal situation may not require separate wires to perform the check function.

The relay here designated TPR represents the sum total of the wrap circuit, which is not itself shown here in detail.  Observe that Figure 10 bears some resemblance to Figure 7.  In this particular case the territory is dark, so the track circuits used by the wrap are designed specifically with the crossing in mind, and the detection area is no larger than the approach lengths as defined by the track shunts.

Circuit Print of Motion Detector
Figure 10--Simplified circuit print for a motion detector used with a stick circuit.

This crossing is unusual because of the situation presented by the stick relay, MDSR, and the 1-minute time element relay, TER.  Here's how it works:

When a train enters the approach (here the same as the wrap), TPR drops out and breaks the control of the XR through the top branch of the circuit.  At this point, the energization of the XR (or lack thereof) depends on lower branch of the circuit through contacts of MDR and MD-IL.   The train causes the motion detector to sense motion and drop MDR, opening the lower branch as well.  The crossing is activated.  If the train should stop on the approach, the MDR would pick up again and tend to energize the XR.  However, in this particular design the supply of battery hasn't been established yet in the lower branch because it comes through contacts of MDSR and TER, so the XR stays down.

MDSR will pick up if the train is moving when it reaches the island, although the XR won't be energized immediately because MD-ILR is down at that point for the train being on the island.  TER will pick up if the MDSR doesn't energize after one minute of the train being on the wrap.  In summary, the wrap basically controls this crossing until the train reaches the island, unless the train has once been on the island already or the train has not reached the island after a full minute of being on the approach.  This curious behavior somewhat approximates ESR and WSR stick circuitry for complete through movements, although it behaves as a motion detector when the train is performing switching operations.

Time element relays are sometimes used for momentary loss of shunt protection.  Another clue that might help to describe why this circuit behaves the way it does is that the model of motion detector used in this design had trouble detecting trains at speeds less than 3 mph.  Train crews complained that they were on the island before the gates came down.  They needed to increase speed to increase safety!  Anyway, the pickup circuit for MDSR tends to verify that the motion detector is working under the given conditions before allowing the motion detector to energize the XR.


  1. Lots of helpful conversation provided by Jerry S.

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