Switches, by design, have dead spots, where power, to a locomotive, is applied through one, not both contacts, with the third rail. On occasion, roller/contact pick-up, can find two dead spots at one time, and the engine stops. There are power routing solutions to this problem, both Atlas and Ross have work-arounds (previously mentioned in this thread, page 2).
One of the big improvements was Atlas providing (4) roller pick-ups on new models