The following sections expand upon the railroad relays/circuits discussed in Chapter 4 and specify how these circuits shall interact with the traffic signal system. The combined circuits are specified in the preemption design and are referred to as the “Interconnection.” The designer shall request the circuits from the railroad to meet the needs of the specific grade crossing location as determined by the diagnostic team.

The Advance Pedestrian Preemption (APP) circuit may be included to extend the Total Approach Time (TAT) beyond what is allowed by the AREMA 50 second rule and provide pedestrian clearance. When an APP circuit is specified, it should always be in conjunction with an Advance Vehicle Preemption (AVP) circuit.

The railroad may provide this circuit to notify the traffic signal controller of an approaching train prior to AVP and prior to activation of the railroad warning devices. The time interval between this notification and the instant when the AVP circuit is activated is known as the Advance Pedestrian Preemption Time (APPT). The Advance Pedestrian Preemption (APP) circuit should be single break.

When the APP circuit is activated, the following sequence shall occur:

The AVP circuit is only used when APP operation is specified. This circuit notifies the traffic signal controller of an approaching train prior to activation of railroad warning devices. The period of time between this notification and the instant when the railroad warning devices are activated is known as Advance Vehicle Preemption Time (AVPT). In most cases, AVPT should be used by the traffic signal controller to terminate any active non-track clearance vehicular movements and to transition to a programmed track clearance interval to clear the track(s). The AVP circuit should be single break.

When this input is activated, the following sequence shall occur:

This circuit is used when APP operation is NOT specified. Where used, the railroad may provide this circuit to notify the traffic signal controller of an approaching train prior to activation of the railroad active warning devices. The period of time between this notification and the instant when the railroad warning devices are activated is known as Advance Preemption Time (APT). In most cases, APT is used by the traffic signal controller to terminate any active non-track clearance movements and to change to a programmed Preemption Clearance Interval to clear the track(s). The Advance Preemption circuit should be single break with supervision.

When this input is activated, the following sequence shall occur:

The supervision circuit is provided through a railroad relay back contact and monitored by the railroad interface for a fault condition in the interconnection cable. The supervision circuit is the inverse operation of the circuit being supervised. If the circuits are both energized or are both de-energized at any time, this shall indicate a vital interconnect failure and shall cause the traffic signal controller unit to transition to all-red soft flash until the fault is repaired. When the Advance Preemption circuit is specified, it should be the supervised circuit; otherwise, the Crossing Active (XR) circuit should be the supervised circuit.

The railroad may provide this circuit to notify the traffic signal controller of an approaching train at the same time the railroad warning devices activate. This circuit is commonly referred to as an "XR" circuit by the railroad. It is also the circuit typically used for “Simultaneous Preemption”. The XR or equivalent circuit should be single break. The XR circuit shall be connected in such a way that the Simultaneous Preemption Relay can never be down with the XR relay up. This is commonly circuited by having the XR relay as a repeater of the preemption relay.

Where Advance Preemption is used, the traffic signal controller should enter the preemption sequence during the APPT, AVPT and/or APT period for through train moves. However, where a train restarts within the approach circuit to the crossing, the APT may be reduced or eliminated. This commonly occurs where railroad switching operations take place, where trains meet or pass, or where trains stop at stations within the approach circuit to the crossing. The railroad then has operating rules which govern the movement of trains over the crossing. However, in these instances where APT is reduced or eliminated and the Crossing Active input is activated, any remaining walk, pedestrian change or minimum green time shall be truncated to zero. Yellow change shall begin for any phases other than the Preemption Clearance Interval phases followed by the red clearance interval. The Preemption Clearance Interval shall start immediately following completion of the red clearance interval and shall continue until the gate down input is received and the minimum Preemption Clearance Interval has been served. The crossing active circuit should also be used to activate blank-out signs restricting movement towards the crossing during preemption.

At locations without gates, this circuit should be used to terminate the Preemption Clearance Interval. Once the crossing active circuit is energized, the traffic signal has at least 20 seconds to complete the Preemption Clearance Interval and transition to limited service. If the advance preemption circuit has already energized, the signal should already be transitioning to the Preemption Clearance Interval. The Preemption Clearance Interval should extend beyond receipt of the crossing active input until the minimum Preemption Clearance Interval has been served.

Where Simultaneous Preemption is used, the traffic signal controller shall transition to the Preemption Clearance Interval as soon as possible. The following sequence shall occur:

If gates are present at the grade crossing, the railroad may provide this circuit to notify the traffic signal controller when the gate(s) controlling access over the track(s) approaching the intersection is/are almost horizontal. The traffic signal controller unit shall not leave the Preemption Clearance Interval until the gate down circuit is closed indicating that the gate(s) is/are down confirming that vehicles are physically restricted from entering the crossing area. Once the gates are down the traffic signal must finish clearing vehicles that have already entered the crossing area.. The traffic signal controller shall extend the Preemption Clearance Interval until the gate down input is activated.

The Gate Down circuit should be single break. If more than one gate controls access over the crossing approaching the intersection or if exit gates are present at the grade crossing, then all mechanisms must indicate that they are lowered prior to closing the Gate Down relay. All gates on approach to an interconnected traffic signal should be connected to the Gate Down circuit to provide the appropriate Gate Down information. In accordance with AREMA 16.30.10, the Gate Down shall be wrapped by the Island relay. This will provide a fail-safe Gate Down indication to the traffic signal controller in the event one or more of the included gates is not down and the train occupies the Island circuit.

This circuit may be provided by the railroad if requested and will notify the traffic signal controller when a train has physically entered the limits of the grade crossing. This is referred to as the “Island”. This circuit may be used to terminate the Preemption Clearance Interval in the absence of a gate down indication as noted in the discussion of the Gate Down circuit. However, by the time the island circuit is energized, the train is already at the crossing. Consequently, a separate island circuit is only applicable in unique situations at locations where there are no gates present. The Island relay circuit should be single break.

The traffic signal controller shall notify the railroad warning system whenever the traffic signal has entered conflict flash, manual flash, soft flash, manual signals off, or when commercial power and backup power system has failed (signals off) through a traffic signal health circuit. The railroad may, at their option provide a traffic signal health relay which will normally be energized by this input. Whenever the Traffic Signal Health relay is down, the operation of the railroad warning devices shall be extended by the Advance Preemption (and APP, if used) time. The Traffic Signal Health relay must remain open until the XR relay closes.