Grade Crossings...

[mbshaw]

This is a question that has been plaguing me for as long as I can remember. Maybe it belongs in the Trains forum but I thought I would ask it here anyway. And maybe it is so simple that many of you will laugh, but again I'll ask it anyway.

How are the bells, lights, and crossing arms activated at a grade crossing. Is it a "highly" sophisticated electrical engineering marvel or is it just simple first-grader reasoning? I mean a train approaches from the east and bells and whistles go off long before the train crosses and they stop shortly after it passes. And the same holds true if approaching from the west. I just don't get it. With today's technology I can easily figure this out how this can be done but 30 years ago?!

Thanks....

[GoleyC]

Very simply done,

The train approaches the crossing and make contact with the single contacts, thus telling the single to go down. However, sometime the single goes down earlier and sometimes it goes down later.

Here's why: A train approaches a crossing at speed of 45mph. He hits the contact, which is activated by taking current from one side of the rail, through the wheels and axle, over to the other side making a complete connection. Now, each pass of the wheels alerts a box letting it know how fast the train is approaching the crossing. If the train is approaching slowely, then the single will go down later. If the train is approaching fast, then the single will go down earilier to compensate for the quick arrival to the crossing.

Once the train passes the crossing there are a set of those same contacts. Those let the singles know when the train as cleared the crossing working in the same manner, they activate the same time the train first makes contact with them in the same manner, but looks to see if this is either the first contact or the last contact.

Does that help?

[mbshaw]

I thought it was something like that. It must be a low voltage system then? Is that why there are wires "rail joiners" joining the track sections?

[Anonymous]

The above reply is a good description, but I was totally lost until I replaced "single" the word with "signal" everywhere. (Not critizing, just hoping to save someone else my confusion!)

[Sperandeo]

In fact the train doesn't contact anything except the rails. Grade crossing signals use track circuits just like those that control block signals.

The track circuit sends power through the rails to hold a relay closed. When a pair of wheels bridges the two rails they act as an electrical shunt –  a short circuit – that cuts off the power to the relay and causes it to drop open.

All the logic required to operate the crossing signals was worked out years ago with relays activated by track circuits. If this relay opens before that one, say, it shows that the train is coming from the east, or vice versa. The direction determination triggers other relays to operate the signals in an east-to-west or west-to-east sequence.

The length of the track circuits determine how long in advance of the train's passing the signals are activated. When a railroad wants to run faster trains on a given line it has to lenghen its grade crossing track circuits.

There are track circuit boundaries – insulated rail joints! – on either side of the crossing, so the relays can turn the signals off shortly after the last car is off the crossing.

So long,

Andy

Andy Sperandeo
MODEL RAILROADER Magazine

[BR60103]

Multi years ago, MR published an article on building a box to control the grade crossing signals for a double track line so that they would stay on if a second train came through before the first left.
It involved a Y shaped piece on a pivot and the relays dropped weights on one side then the other. Took me a long time to figure out how it was supposed to work.
--David

[mbshaw]

Thank you all for helping me understand. Just one of many enigmas that I ponder.