The MU cable doesn't control the engine, but provides power for the lights.
Using the plow moves the snow farther than an engine plow pilot. The pilot must be 3" above the rail, per Fed. regs. Flanging out below the rails is most important. When the snow is over the rails, it is hard to control the train. It gets on the wheel treads, and reduces the friction of the brakes. The trick is to warm the brakes up well before the hill. Sometimes that doesn't even help. I never had one get away, but I had nearly full service on to control the train. A friend said that he couldn't hold a heavy train coming down through Westminster. He had District 3 line him up through Fitchburg.
My train was heavy, but also had lots of salt cars. They have some new aluminum ones, but most are old cars. The salt is so corrosive that they use cars on them last legs. Most brake poorly even in good weather.