What makes the gates go down?

Being a commuter rather than working for the railroad, I've often wondered what triggers the gates to go down at grade crossings?

Frankie

Short answer: Circuitry in the track. When a train comes it shunts the track circuitry causing the gates to go down.

I'm sure someone here can give you an in-depth answer though.

...and that circuitry is very simple, with its design going back to the 1800's. Electric portions and diesel portions of the railroad are handled slightly differently, but look at it like this diesel territory circuit:

A 48 volt battery is placed across the rails, with the ( + ) side on one rail and the ( - ) side on another. Further down the track you have the device connected to the rails that holds the gate in the up position. It's actually an electromagnetically operated "swing dog" that holds up the gate.

When a train comes along, the steel wheels and axle short out or "shunt" the battery circuit, dropping the voltage to zero.

When that power drops, the electromagnet holding the dog releases, and it allows the gates to drop to the down or normal position.

The track is broken up into different blocks with insulated rail joints isolating them from one another.

When the train passes, the gate is raised by an electric motor.

In practice, the interaction of three blocks adjacent are involved, with one being on the crossing, and one on the approach in each direction. The interaction of the three permit the circuits to determine the direction of the train, lower the gate in advance of the crossing and raise them immediately after the train clears the roadway.

This system also drops the voltage if a rail breaks and is the same simple signal circuit that operates signals.

It's simple, inexpensive, fail safe and nearly impossible to trick.

The distance the circuit is set, relative to the crossing, as I recall, depends on the type of service (passenger/freight) and the max speed expected. The max speed in turn is driven by condition of the track...

Interesting, how far down from the crossing do they usually place these trips? I assume a few hundred to a thousand feet or so, I'll have to be on the look out for them next time I'm waiting for a train.

hammerfang wrote:Interesting, how far down from the crossing do they usually place these trips? I assume a few hundred to a thousand feet or so, I'll have to be on the look out for them next time I'm waiting for a train.

The distance varies based on the speed of the line. The design is such that the lights are enabled and the gates drop with sufficient warning before a train enters the crossing. This would place the trip point half a mile away for a 30 second warning on a 60 MPH line. That point would have to be further away on faster lines for the same warning time or could be closer to the crossing on a slower line.

On signaled railroads the blocks used for triggering crossings are part of the signaling system. The shorting detection system noted earlier in this thread is used to detect something on the rails within each block. At the end of the block the rail is cut to isolate one block from the next (a short across the rails on one side of the cut is detected as a different block as the other side of the cut).

For a simple crossing with no crossings for miles you will find an "island" circuit that extends across the crossing for a short distance on either side (distance varies) and an approach circuit in each direction. Then additional circuits beyond the approach that don't affect the crossing in question. All of the circuits are tied together via relays to create a block for the signaling system (if one exists). If crossings are close together (for example, a quarter mile apart) the system gets complicated with overlapping approach and island blocks in different directions.

Probably your best chance of seeing the end of a circuit for yourself is at a crossing. Do some railfanning (staying OFF of railroad property). LOOK for where the train is when the lights and gates activate and use a watch or stopwatch to discover how much warning is being given. LOOK for where the end of the train is when the gates start going up again. The end of the island circuit will be between the point where the end of the train was and the crossing. If that point is visible from non-railroad property you should see the cut in the rail. (Don't trespass to see the cut. These cuts in the rail are also present near signals and interlockings. I've seen a few from station platforms where the public is expected and welcome. There are plenty of places to see infrastructure without trespass.)

A quick search found this http://media.photobucket.com/image/rail ... G_2012.jpg . The gap will be filled in with non-conductive material.

I'm surprised that no one has mentioned it yet, but there is also a type of crossing technology called "predictor" which senses the speed of the train and activates the crossing gates/signals accordingly. In other words, it can sense the difference between an underpowered freight train doing 20 MPH and a passenger train doing 79 MPH. Usually they're configured so that the gates come down 30 seconds before the train arrives at the crossing, regardless of how fast or how slow the train is going.

For more info, see http://en.wikipedia.org/wiki/Grade_crossing_predictor

MODERATOR'S NOTE:

Thank you for your patience. The thread is now open again

So what happens if an engine happens to go by a crossing completely out of the blue and unscheduled, gates are still activated?

And how come when those pick-up trucks on rails goes by, the gates don't come down?

airman00 wrote:So what happens if an engine happens to go by a crossing completely out of the blue and unscheduled, gates are still activated?

Yes. Gates are triggered by detection of a train. If all works any train, scheduled or not, should trigger them.

And how come when those pick-up trucks on rails goes by, the gates don't come down?

They are designed not to trigger detection. They run on their rubber tires with the rail wheels used only to guide the vehicle, not to short across the rails like a normal train car or engine's axle. The electrical connection needed between the rails to detect the car isn't made (unless the wheels are shorted together - which can be done for circuit testing).

There are also manual override switches located at or near crossings, so if the train has cleared the crossing but not the sensor to raise the gates, a crew member can use a key to operate the gates, and to lower them when necessary, such as when a maintenance crew is working.

What keeps the gates up when a train is stopped within range to operate the gates? For instance where the trains have to stop for a signal on the Lehigh line here in Edison N.J. on Tingley lane is way closer to the crossing than the distance that normally operate the gates, so what's involved in holding them up until needed when the train finally gets going?

davebdawg wrote:What keeps the gates up when a train is stopped within range to operate the gates? For instance where the trains have to stop for a signal on the Lehigh line here in Edison N.J. on Tingley lane is way closer to the crossing than the distance that normally operate the gates, so what's involved in holding them up until needed when the train finally gets going?

An electrical interlock in the nearest signal bungalow that keeps the gate circuit deactivated until an other-than-stop signal is displayed and the train gets moving again..

It's not magic, it's engineering.

MJK

justalurker66 wrote:

airman00 wrote:

And how come when those pick-up trucks on rails goes by, the gates don't come down?

They are designed not to trigger detection. They run on their rubber tires with the rail wheels used only to guide the vehicle, not to short across the rails like a normal train car or engine's axle. The electrical connection needed between the rails to detect the car isn't made (unless the wheels are shorted together - which can be done for circuit testing).

Some newer hirail trucks have equipment installed that can short the circuit with a switch in the vehicle and make the gates lower.

http://www.youtube.com/watch?v=9MrUTNKu9Uw