How does a train (in the US) set off the warning signals at a crossing?

Question

How does a train (in the US) set off the warning signals at a crossing?

Answer 1

At some commuter rail stations, there are also "horn activated" crossing signals.  If a train is slowing into the station, the signals DON'T activate.   Then when the train is ready to go, he sounds a blast of his whistle, and the gates & flashers kick on!

You can spot these installations by what looks like an old-style P.A. speaker on a pole - it's the horn sensor.

Answer 2

you will not find any better answers than hoghead and budgies

Answer 3

It runs over a switch right before getting to the crossing...

Answer 4

It is done automatically and electronically.  

There is a "bug," as it is called, that is located dependent on the track speed.  The higher the speed, the further the distance of the bug from the crossing.  They are usually placed far enough in advance for the gates to activate and lower 15 seconds before the trains arrival at the crossing.

Many are equipped with a "time out" feature that comes in to play when a train has stopped after initiating the sequence.  After a predetermined time, the gates will raise.  When the train again begins to move, the gates will then re-activate.

Some have to be restarted manually, however, while others are equipped with a microphone that will re-activate the sequence when the engineer blows the whistle when starting to move.

But, there are a couple of inherent dangers.  The primary concern is that these warning devices are NOT fail safe, so when approaching a crossing, LOOK and listen for an approaching train.  Your life as well as the lives of your loved ones depends on it.

When a train or cut of cars are left standing near a grade crossing, they are to stop back at least 150 feet, if practical.  If there are two tracks, it should be posted, but you can't always count on that either.  The problem in this scenario is the motorists or pedestrians see the stopped equipment while the warning devices are activated, and assume there is no danger, while the train that is going to kill them is running on the other track, with vision obscured by the standing equipment.

So take a moment and safeguard your crossing.

Answer 5

Track circuits, something in use all across the railway system, are what set off the crossing lights.

Sections of rail are electrically insulated from one another, and a small current of electricity is passed through them.  Since the wheels of locomotives and rail cars are steel, they create a short circuit, which trips any number of circuits in a signal bungalow to do a variety of functions.

Most common is to set off crossing warning devices.  Usually, the "track circuit" is located away from the crossing at a distance dependent upon track speed, that will allow the devices to be active for at least 20 seconds before the movement occupies it.  Sometimes, on industrial and side tracks, the circuit is very close to the crossing, so the train needs to stop completely, or crawl up to the crossing to set off the lights.

These same track circuits that set off the crossings are also used in conjunction with the signal systems, dispatcher panels, and computer guided hump switching operations.