Railroad Preemption Wired to the Island Circuit
An intersection is 90 feet from a highway-rail grade crossing on a 60 mph rail line. The traffic signal is interconnected with the railroad's warning system. The railroad's approach circuit extends 3,000 feet from the crossing; the island circuit extends 800 feet. The signal's preemption sequence requires 20 seconds of right-of-way transfer time plus 8 seconds to clear the 90-foot queue area between the stop line and the far rail. During a test with a train, the signal enters preemption, but the train arrives at the crossing while the track-clearance green is still active and two vehicles are stopped on the tracks. You find that the cabinet's preemption input is wired to the railroad's island relay, not the approach relay. The railroad's signal maintainer says the approach relay is available but the contractor did not request the wire pair. The traffic signal is otherwise operating normally.
1. Calculate the warning time available from each railroad circuit and compare it with the required clearance time. Show your work.
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Convert the train speed to feet per second: 60 mph × 1.467 = 88 feet per second.
- Approach circuit: 3,000 ft ÷ 88 ft/s = 34.1 seconds of warning time.
- Island circuit: 800 ft ÷ 88 ft/s = 9.1 seconds of warning time.
The required time is the right-of-way transfer time plus the track-clearance time:
- ROW transfer: 20 s
- Track clearance: 8 s
- Required total: 28 s
The approach circuit provides about 34 seconds, which leaves roughly 6 seconds of margin. The island circuit provides only about 9 seconds, well short of the 28 seconds required. That explains the observed result: the signal cannot terminate the conflicting phases, provide pedestrian clearance, and clear the 90-foot queue before the train arrives. The preemption input must be taken from the approach circuit, not the island circuit. The island circuit is intended to detect a train occupying the crossing area, not to provide the advance warning time needed for traffic-signal preemption.
2. What is the correct preemption interconnection, and what must be done before the next test train?
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The correct interconnection is from the railroad's approach relay in the bungalow to the traffic-signal cabinet's preemption input, using the railroad's standard interface and the agency-approved circuit. The island relay should not be used as the advance preemption source. The interface must be fail-safe: loss of the preemption circuit, a broken wire, or a relay failure should cause the traffic signal to enter preemption or otherwise revert to a safe condition, not suppress preemption.
Before the next test train:
- Obtain railroad approval and coordinate with the signal maintainer; the contractor must request the available wire pair and install it per the railroad's requirements.
- Verify the relay type, contact configuration, and voltage at the cabinet; confirm the input is compatible with the controller's preemption input.
- Perform a wire-break and relay-drop test to confirm fail-safe operation.
- Measure the actual time from preemption call to track-clearance green, and from track-clearance green to the train's arrival, during the test.
- Confirm that pedestrian phases crossing the tracks are terminated and that the track-clearance phase is correct.
- Have the railroad and the agency witness the test and record the results before restoring normal operation.
3. What fail-safe checks and records are required before the intersection is placed back in normal operation?
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The fail-safe checks should confirm that the signal does not depend on a single energized circuit to provide track clearance. Verify that a loss of the preemption input, a broken wire, or a relay failure results in preemption or a safe flashing condition, not normal operation. Check the controller's preemption programming, the pedestrian clearance intervals, and the track-clearance phase assignment. Confirm that the railroad's warning devices and the traffic signal are coordinated and that the test train produced the expected sequence.
Records should include the railroad's approval of the interconnection, the revised cabinet wiring diagram and as-built, the preemption timing measurements, the relay and wire-break test results, the test-train log with date and time, the names of the railroad and agency witnesses, and the MMU event log. The agency's signal file should be updated with the final preemption configuration. Do not place the intersection back in normal operation until the approach relay is connected, the test train has verified the clearance time, and the documentation is complete.