Crossing Flashers Explained

I’ve never understood crossing flasher behavior in industrial settings.  Regardless of location when a train approaches a crossing at speed, industrial or main line, the behavior is simple enough.  A predetermined time/distance away the lights activate and the gates drop.  After the train passes a few moments later everything resets to normal.  Not so during industrial switching.  I’ve seen numerous occasion where the train is a few feet from a busy intersection and the lights are off and gates are up.  As soon as the train crawls a few feet the circuit activates, the lights flash and then the gates do drop.  I never understood how this happened.  Did the crew have a switch to turn things on and off?

To the rescue comes Paul S., a professional signal maintainer by trade, who was gracious enough to explain it all.  He writes:

Hello Lance, I just viewed the video you ‘d linked to (skip to the 3:30 mark) of the Downtown Spur Switcher.  The crossing activation system was functioning as intended. This crossing is equipped with either a predictor  or a motion detector for activation. A predictor activates a crossing when a train is a pre-set time from the island — that’s the part of the crossing that intersects with the street. FRA minimum warning time is 20 seconds. We typically use 30 seconds on the (deleted). Regardless of a train’s speed, up to the timetable maximum mph (which determines another parameter we input into the predictor’s CPU), the predictor will always activate the crossing when the train is 30 seconds outside the island. However, if a train stops in the crossing approach outside the island, as the Downtown Spur switcher did while the switch was being lined from main to the spur, the crossing will recover after a pre-set interval as shown in the video. After this recovery, however, the predictor switches to motion-detect mode. As soon as it senses “motion” the predictor re-activates the crossing immediately.  Whenever a locomotive or railcar is occupying the island, the crossing will always activate. The island is a separate circuit of the crossing. “Positive island ring” is an FRA requirement.A motion-detector is basically a complicated on-off switch that activates the crossing regardless of a train’s speed. Once the train is in the approach and the MD senses its presence, the crossing activates. Usually MDs are used today only where speeds are slower and approaches shorter, such as on a branch. If a train stops in the approach to the island, the MD also recovers, and then reactivates upon a decrease in AC voltage on the rails.I’m a signal electronics technician for (deleted)