More on light rail braking, which was a surprisingly hot topic – I didn’t realize that dynamic braking on light rail cars would be that interesting in the blogging world, but there we are. Anyway, a belated hello to the people coming over from Reddit.
Disc brakes, also known as Friction brakes
In my last post I mentioned that dynamic braking is the primary method of braking at speeds greater than 3 miles per hour. Slower than 3mph, the dynamic braking blends with the friction brakes. This is because at speeds that slow, the motors-acting-as-generators can’t generate enough power to actually stop the train – remember that dynamic braking works by converting the motion of the train into electricity. Not enough motion = not enough stopping power. So at about 3mph, the friction brakes are applied, and ultimately these stop the train once the dynamic brakes have slowed the train enough.
The friction brakes are located underneath the train on the wheel axles.
Friction brake on a wheel truck in the shop – it’s the part that kind of looks like a sideways Coliseum in the middle
Here is a better un-blurred picture of the friction brakes. Actually all of the photos in that set are worth looking at if behind-the-scenes train stuff is your thing, and considering that you are presently reading a blog entry about the braking systems of light rail vehicles, it probably is your thing.
Inside the cab, the operator can tell if the friction brakes are applied or released with this indicator light, found in Types 1-3. It’s lit while the train is in motion above 3mph because the brakes are released. This indicator goes dark only when the brakes are applied. In the Type 4s, a similar indicator is lit only when the brakes are applied, and is dark when the brakes are released.
If you’re outside the train or in the cab looking at the mirrors, you can tell when the friction brakes are applied by watching the red brake indicator lights above the wheel trucks (Types 1-3) or over the doors (Type 4).
In this animated gif of a train stopping at a platform, you can see the brake light which is above the second passenger window from the door come on as the train almost comes to a stop, then it goes dark again when the brakes are released as the train moves up a bit (probably because the operator stopped on the dead spot) and then back on as the train comes to a final stop to service the platform.
On a Type 4, the exterior brake indicator lights are located right above the door open indicator lights – the red light is the brake indicator and the yellow light is the door open indicator. Looking at a stopped Type 4 when the doors are closed, only the brake indicator lights will be lit.
The exterior brake indicator lights will be dark if no operator is keyed in to the train even though the brakes will still be applied. This can be a useful thing for passengers to know – if you’re running to make a stopped train at the end of the line and you don’t know when it leaves, check the brake lights. If they’re dark, the operator hasn’t keyed in in the cab (and might not even be in the train yet) so you still have some time. If they’re lit, the operator is in the cab so the train will be leaving shortly.
Friction brakes work on a hydraulic system. Occasionally a friction brake will “hang” and will be stuck applied. When this happens, the operator can manually release the brake by pumping off the hydraulic fluid. This is what those boxes labeled “MRU” or “brake release unit” inside the trains are for – this is the manual release unit. The whole procedure for releasing a friction brake is very different in the Type 4s – you can see some of it (how the brakes are pumped off) in this video from earlier this year when a Type 4 had mechanical problems at NE 60th.
This one I have mentioned before – the track brakes hang between the wheels. The operator applies this to assist in stopping the train, typically on slippery tracks. This is also the brake that will be used in case of emergency during a dead car push. Because it rapidly brings the train to a stop which can be jarring for passengers, it’s not used in normal platform service unless the slippery condition of the rail warrants it.
Trivia: From a speed of 55 miles per hour, a MAX train will take about 600 feet to stop.