So, I started up a GP38-2 from cold in the free switching scenario on NEC and then started to realise my train brake wasn't working. I thought I had set up everything correctly from memory but had obviously forgotten something. I then realised I hadn't set the cut-off valve to freight. I skimmed through the manuals but didn't see a simple explanation of what this and the mu-2a valves actually do. I did a google search and found this info in a reddit post which you guys might find informative also (if you're like me and not a real life train driver):
In addition to this, the cut-out position stops the compressor from topping the air in the brake pipe. This is how you can test the system for leaks. You charge the brake pipe 90psi, then move the cut-off valve to the cut-out position and watch the pressure gauge. The reduction in pressure over the course of a 1 minute period should be less than 5psi, any more than 5psi and the brakes may not function correctly. Switching to passenger mode will also charge the brake pipe to 110psi instead of 90psi in freight mode.
Interesting, thanks. Why the difference in pressure for passenger vs freight? Just wondering what the reasoning is behind it.
I'm guessing in case one had to "dead head" a passenger train for say the engine were to malfunction.
With it being 110psi, the reservoirs on each vehicle are charged to 110psi, when pressure in the brake pipe drops (the driver applies the brake) a valve can allow the 110psi to act on the brake cylinders (which push a shoe/pad against the wheel), this effectively means that a passenger train can brake harder than a freight train, which is what is required for passenger operations where the trains are shorter and stopping quickly saves time. A freight train will only have a maximum of 90psi acting on the brake cylinders, so 20psi less pressing the shoe/pad against the wheel or brake disc of the freight car. With a freight train you don't want the brakes to come on as hard, because freight trains are much longer than passenger trains and the brakes at the front will apply faster than the others at the back. If the brakes were applied too hard, the end of the train (unbraked at this point) would pile up against the front of the train (brakes on at this point) and potentially cause a derailment. So there is a safe limit to how hard brakes can be applied. You could run a passenger train at 90psi and the brakes would still function, however the stopping distances would be longer.
