How do trains activate the upcoming Railroad Crossings?

Question

simple question....

How are the lights and gates activated as a train approaches an intersection?

I have my own theories, but I'm wondering if somebody can give me the real answer.

Answer 1

The first two guys are right. I talked to a RR Signal Maintainer and he said the amount of current in each rail is only about 1/3 of a volt. When a train approaches the signal, a micro processor calculates the speed of the train and activates the gates accordingly. With current in each rail, on either side of the crossing, the approaching train actually electrically shorts-out the current, activating the system.

Answer 2

hmmm.... I didn't know it was all so detailed. I thought they still had the little guy in th hut beside the track that went out and put his ear down to the track to listen for approaching trains.

If it's so micro processed now then why do the lights flash and arms lower when there is no train? hmmmm... next question maybe?

Answer 3

Well, I am a controller of these crossings in the UK. (A signaller).

There are various ways of activating a the closure of a crossing over here. The most simple one is the guy who presses a button to lower them. As the guy knows exactly where the train is, he knows when to close the gates. The other on is a treadle. These are triggers placed on the side of a rail and when a train wheels pass over the trigger, the gates lower, activate. Each wheel is counted at the trigger, telling the crossing how long to stay closed, all this is dependant on the line speed.

And the other has already been listed as an answer. The one about the electrical current. The train makes up the circuit, activating a relay which tells the crossing to close.

Answer 4

Consider the times you live in.  Railroad signaling is interesting, but I'm skittish about giving out technical details in this day and age.  

Sure, someone might use the knowledge for malice... but I'm far more concerned that the authorities might mistake someone's innocent curiosity for malice.  Their track record on the treatment of plainly innocent railfans isn't encouraging.

Why do they go off inadvertently?  In general terms, because they're designed "fail-safe", to err on the side of caution.  It's less bad to go off when there's no train than to NOT go off when there's a train!  Rain, or salted road runoff is enough to set some of them off!

In any case there's never an excuse for not looking both ways.

Answer 5

The rails leading up to the crossing are insulated from the ones behind them, about 1/2 mile before the crossing.  When the engine... or a car... passes over the rails, it completes the circuit and activated the relays that operate the gate and light.

It'a current flow system and not a voltage flow system.

Answer 6

Detection of a locomotive or any common railcar is simple, as stated in the answer above, however, there is more to it than just a simple relay.  Today's controllers are very complex, using microprocessors for detection and predicting when the train will arrive at the crossing and triggering the flashers at the proper time.

Even before microprocessors, there were elaborate detection circuits that produced accurate prediction functions for flasher activation at the proper time, as you don't want a train going 2 MPH to activate a signal a half mile away, although it may seem like it when you are the one waiting for the train.

The secret to properly predicting when to activate a flasher is achieved using Ohm's Law and the resistance of the rails.  Even though the resistance is EXTREMELY low, the rails still have enough resistance to provide train location data to an analog or digital circuit.  The rate of change of the resistance accurately tells the circuit how fast the train is moving, hence, the circuitry is capable of making a decent decision of when to turn on the flashers.

One site you may find interesting:

http://matt.zont.org/signals/crossings/x...