How do train signals know a train is coming?

Mark Brooky • Jul 21, 2015 at 12:26 PM

Eric further asked, "Are there weight sensors under the track, or does the conductor press a switch in the caboose? Why do they sometimes flash when there is no train around and what should be done if I see that? Who makes sure they are safe? Can I plant flowers around the lights in my frontyard?"


Whoa, that's a lot of questions, Eric. Let me try to get you back on track!

There's a Tribune online commenter who goes by the name SignalMaintainer who obviously works for the railroad. I got in touch with him, and he offered to help answer Eric's question, as long as he remains anonymous (citing company policy and Department of Homeland Security reasons).

It gets a bit technical, so hold on to your pocket protectors and get out your slide rules!

"There are three electrical circuits that are made with the rails themselves on each track at the crossing — an island circuit (the bit of track in the roadway itself) and the two approaches (on either side of the crossing)," SignalMaintainer explained. "The approaches range from a couple hundred feet long to a couple thousand feet, based on the track speed limit. Federal law requires we provide at least 20 seconds of warning for motorists before the train touches the edge of the road. All the circuits are separated by insulated rail joints — in most cases."

According to SignalMaintainer, many people believe a train completes an electrical circuit, and thus turns on the lights. But the truth is actually the opposite. He explained that it's more like a DC (direct current) circuit, in which a relay is constantly energized by a battery and held up by electromagnetic forces. 

"When a train enters the approach, its wheels actually short-out that circuit, and the electricity doesn't make it to the relay at the road," he explained. The relay loses energy and "drops," which causes a set of contacts to touch, activating the signal lights through a series of relays.

"The reason it is set up this way is to provide fail-safe operation," SignalMaintainer said. "If there is a problem anywhere in the track circuit — which could range from a broken wire or rail, to something as simple as snow or heavy rain causing too much of a draw-to-ground on the circuit — the crossing will automatically turn on, and stay on until the problem is fixed. It doesn't do that to annoy people or cause traffic back-ups — it does that to protect you and provide warning that something is wrong."

He said the rest of the operation gets even more complicated and technical.

Newer technology uses motion detectors and predictors. SignalMaintainer said they can shut off the crossing if the train stops before the crossing, and can provide a constant warning time no matter the speed of the train.

"If the train is going fast, it will turn the crossing on when the train is farther from the crossing, or much closer if the train is going slow," he said.

If you ever see a train crossing activated with no train around or other emergency such as a car stuck on the tracks, SignalMaintainer urges you to call the phone number located on either the light mast or signal cabinet (it will be on a little white or blue rectangle) and report it. The dispatch center will want to know a seven-digit "crossing ID" number, as well as the road name and city. They will then send a crew to check it out.

By the way, tampering with or vandalizing a railway signal or related equipment is a serious federal crime, SignalMaintainer notes, and violators may face terrorist charges.

"There has been an increase in vandalism to the crossings through town in recent years, and it puts the general public at risk," he added.

As for planting flowers around signals, SignalMaintainer suggests contacting the railroad offices (Genesee & Wyoming RR in this case) and discuss it with them.

"I will say, somebody on Pennoyer Avenue in Grand Haven always plants a nice flower bed around the signal," he said. "I feel bad though, since I frequently need to set up my ladder in the middle of them to access the lights. Maybe it's not the best idea, as nice as they look, to plant directly in front on a signal."

Should you want to dig further into how railroad lights and gates actually work, CLICK HERE.

For info on how a DC track circuit works, CLICK HERE.

And how motion detectors/predictors work, CLICK HERE.

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