How do crossing gates work?

alot of the railfanning vids iv seen usely include trains at crossings but my question is exacly how do the gates work? how dose it know theirs a train comeing?
Is their something in the cab the conducter switch?
Dose it have a sencer some points away from the crossing then the sencers make the gates go down?

thanks in advance.

crossings normally work , due to wheels shorting a electric current in rails.
some work with a aproach and a island circuit, some work with a predictor circuit.

Check the Long Island Rail Road forum under Commuter - Transit - Light Rail. There's a lively discussion on one of the threads that goes into considerable detail about various means of controlling gates.

Try this link:

http://matt.zont.org/signals/crossings/ ... works.html

wigwagfan . Thanks so much for that link!
I really learned alot from the web site and dont know why but added it to my faverits.