How mobile phone signals could cause train crash:

How mobile phone signals could cause train crash: Government investigating safety issue with Network Rail | Mail Online

It will be pretty well impossible to stop people using their phones, computers, i/pads and stuff on the trains. But if a non use policy within a mile of a train track were brought in that would certainly be impossible to police. So if, in fact, a signal can affect a train, then trains must have filters to stop this from happening, very quickly.