What signal? Who said anything about a signal? Other than you, of course.
An array of starter switches can be positioned, one near each car of the train, connected to synchronized timers, so they're all timed to start the engines simultaneously in the ground frame. Then the pre-programmed engines can accelerate their cars, each according to its own previously calculated schedule, with no further input.
As I already mentioned, it's probably a silly and even dangerous thing to do in real life, and I don't think DimaMazin's timing function would work correctly. But there are no signals involved after the initial setting of the timers, so your comments about signals are immaterial.