How do level crossings work?

Question

i need to know the in depth physics and detail of how level crossings work, like how the light knows to turn red when the train is coming??

thanks 4 any help

Answer 1

You want in-depth?  From your profile, you are probably not looking for North American systems, but that's all I have, so here you go:

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

Answer 2

The trick is to ignore the red light and continue through in your tractor. Particularly if you live in Norfolk.

Answer 3

There are sensors built in to the rail, when an approaching train activates these sensors they calculate the speed of the approaching equipment and activate the crossing protection devices.

Each rail has micro currents, and when an axle completes the circuit from one rail to the other it starts the process. The brains of the installation is in the metal "dog house" near each crossing. They are independent of one another and most have battery backup systems so that they will operate for a time in the event of a power outage.

This is the system in most of the States, I can only assume the basic technology is similar in the U.K.

As a long time railroader I cant help myself from getting on my little soapbox and saying that crossing protection are mechanical devices and the best ever built can and do fail. The only way to be absolutely postively safe every time is to LOOK, do not trust a device to do that for you.

Answer 4

Rango is mostly right from a US point of view, but UK crossings are very different (we do have a couple similar to the US, put in as trial installations but with extra circuitry to meet UK standards).

For AHB crossings (automatic half barrier), these are triggered by the occupancy of track sections a fixed distance from the crossing, backed up by treadles (levers depressed by the wheels). This fixed distance allows a minimum time between the barriers coming down and the train passing. These are also the types of crossing that car drivers regularly ignore and end up splattered. The rail signals are not interlocked with the barriers.

For full barrier crossings, either locallly monitored or remotely via CCTV, somebody has to see the crossing and press the "crossing clear" button if it is clear of obstruction. Most require the barriers to be lowered manually by the signaller or crossing keeper (by pressing the Down button). These installations ARE interlocked with the signals and are therefore relatively safe for rail passengers as the chances of hitting an errant car is minimised.