How do railroad crossing warning lights/gates work?

[Jake]

I was just watching an episode of rescue 911 on youtube, where a pair of runaway boxcars collided with a car at a crossing (perfect T-bone too, what are the chances of that??) because the elderly driver and passenger had no indication that the cars were coming. No gates, bells or lights. They just drove onto the crossing and the boxcars hit them. Now I was under the impression that the warning signals were operated electrically. There is a current running between on the rails, and when the car/train drove onto that particular section of track, a circuit would be completed by the wheelset (kind of like with block signaling) thus triggering the warning lights. But, looking at this video, that does not seem to be the case.

Also, the video I was talking about can be seen here

[Santa Fe buff]

I was just watching an episode of rescue 911 on youtube, where a pair of runaway boxcars collided with a car at a crossing (perfect T-bone too, what are the chances of that??) because the elderly driver and passenger had no indication that the cars were coming. No gates, bells or lights. They just drove onto the crossing and the boxcars hit them. Now I was under the impression that the warning signals were operated electrically. There is a current running between on the rails, and when the car/train drove onto that particular section of track, a circuit would be completed by the wheelset (kind of like with block signaling) thus triggering the warning lights. But, looking at this video, that does not seem to be the case.

Also, the video I was talking about can be seen here

You saw that to? I love that show, I watch that and other episodes everyday on You Tube. I think I've heard of more mechanical signals are set off by certain weight only a train would have, possibly the boxcars, although full, didn't weigh enough, it couldve been a malfunction...

[rogertra]

Here's the simple answer.  It's too complicated to go into detail on this forum on how it actually works.

The design of the track circuit is "fail safe".  There is an electrical circuit between the rails that keeps the lights and bell FROM ringing and flashing by holding the activation relay(s) away from the bell and light and barrier activation circuit contacts.  Thus nothing happens.

However, when a train or a locomotive or cut of cars enter the track circuit they create a circuit that by-passes the relay(s).  This causes the activation relay(s) to drop making the circuit that starts the cycle of bells and lights flashing, barrier (if any) lowering and remaining down all the time the track circuit is occupied.

There is also track circuitry that detects from which direction the train, locomotive or cut of cars is approaching the crossing and this is what makes the bells, lights and barriers begin to lift as soon as the last railway vehicle is clear of the crossing.

Keep in mind that a boxcar or two may not trigger the track circuit as they may not weigh enough to create the by-pass circuit to drop the activation relay(s).  M.O.W. equipment such as trackmobiles and speeders for example, will not trigger a track cicuirt either due to lack of weight or because their wheels are deliberately insulated so as not to trigger the track circuit.

Hope this was of help.

[Guilford Guy]

Jake, the same thing happened a few weeks back, when the lumber car rolled away, and into an oncoming commuter train. All the way down the branch, none of the crossings were triggered.

[pdlethbridge]

GG, you might say it was just lumbering along!

[WoundedBear]

Odd indeed....I was under the assumption that railroad air brakes worked similarily to a transport trucks' 121 air brake system.

I was taught that the 121 system was based off the WABCO design and that it takes air pressure to RELEASE the brakes. A failure of a reservoir or compressor would trigger the failsafe and the brakes would apply. At least that's how trucks work.

How could these two boxcars simply roll away on their own? With no mainline pressure, the relay valve should be inoperative and the springs in the air chambers themselves should have the brakes locked on.

What am I missing here?

Sid

[kevin2083]

The air lines can be closed off at the hose connection (the name is somewhere in my head, but won't come out). That way, the cars can be left with the brakes off for whatever reason.

[Jake]

The air lines can be closed off at the hose connection (the name is somewhere in my head, but won't come out). That way, the cars can be left with the brakes off for whatever reason.

IIRC, the term is 'angle cock'.

[Guilford Guy]

They can also bleed the brakes by pulling a pin. (don't ask me the terminology, this info is from my friend Chris who works for a railroad)
Here's a diagram for bouncing a car which requires bleeding the brakes on one car.

The red car has its handbrakes applied, and there is still air in the tank. The blue car has had the pin pulled, and the hand brakes applied. The gray locomotive decouples from the blue car momentarily, and all the knuckles on the blue car, the left knuckle on the loco, and the right knuckle on the red car are closed. The locomotive then runs into the blue car, sending it cruising down the track, until it hits the red car. The brakeman immediately lines the switch, and the blue car rolls on the siding without a need for a runaround.

[rogertra]

Guildford Guy.

What you described is called a "Dutch Drop".

[rogertra]

You guys need to read up on how railway airbrakes work.

Airpressure both keeps the brakes "off" and "On".

However, when switching cars in a yard, it's usual to bleed the air "Off" so that there's no air in the resovoirs.

http://en.wikipedia.org/wiki/Air_brake_(rail)

[pdlethbridge]

then what are retainers for?

[Mike]

Two things: There is no spring to keep the "emergency" brake on a train car as there is on a road trailer brake. The source of power for the "emergency" brake on a train car is one half of the large air reservoir on each car. When that air is bled off (and no other air is applied) the brakes release.
Retainers on a train car keep some air (adjustable amount) on the brake cylinder, even during a release condition. With some air "retained", it doesn't take as long to build up enough air to do another reduction (applythe brakes again) should it be necessary. This can be VERY helpful on a long grade with heavy cars.- Mike

[pdlethbridge]

like saluda

[WoundedBear]

Good article Rogertra.....

and Mike....your explanation has cleared things up as well. Thanks for the lesson. I now see the difference in how the two systems treat both service and emergency applications.

Sid