How do the arms at a railroad crossing know when to go down?

Question

So I know this is a stupid question, but I really do not know the answer. I am assuming that there is a station that times the arms. Or is it something the conducture does?

Answer 1

Sensors on the tracks

Answer 2

The answers about switches in the tracks are right, there are a series of sensors called "anticipators" that have to be triggered in sequence to activate the signals.

These sensors know if a movement stops before it occupies the crossing and will relaese the protection to allow automobile traffic to move again.

A lot of maintanance of way equipment like Alco describes, the rail truck systems have wheels that are insulated from the axles so it is impossible for them to activate the crossing systems. In these cases there are extra rules and regulations placed on the operators of this type of equipment.

It is very reliable system with a high degree of redundancy but it is still a human made device and if you do not LOOK yourself each and every time you are crossing tracks you are placing your very life in someone else's hands.

Your vigilance is the very best type of crossing protection ever invented.

Answer 3

http://en.wikipedia.org/wiki/Track_circu...

this has diagrams

Answer 4

A set distance before the crossing, the train runs over a switch on the tracks that triggers the arms.

Answer 5

The rails are used in a DC track circuit, positive energy on one rail and negative on the other.   When the train starts into the approach to the highway crossing the energy is shorted across the two rails and a track relay drops in the control case starting a chain reaction of events leading to the release of the gates and the illumination of the lights.  That is a simple crossing, most crossings now are controlled by microprocessors but are still track circuits at heart.

Answer 6

there are switches on the rails that get tripped as the train approaches

Answer 7

Not a stupid question at all!  :o)  The short answer is that a train will trigger an electrical circuit (by completing it in electrician's terms) in the track which then sets off the crossing lights and, if present, gates.  

Of note is that not every piece of rail equipment can set off a protected crossing.  It either has to be heavy enough to do so or properly complete the electrical circuit.  The reason for this is so that traffic is not tied up every time rail equipment is within a crossing "zone."  For instance, if maintenance-of-way equipment (particularly a "hi-rail" pick-up truck, which is just a truck that has rail wheels to operate on railroad tracks) is working nearby but not blocking the crossing there is no need for it to set off the warning lights and gates (and would just needlessly be holding up traffic).