TCAS has solved this for airplanes. It wouldn't take much to modify the system for trains so that, in the least, it notifies when an unexpected train is nearby. A little more logic could allow it to verify which track each train is on.

Obviously it wouldn't be "just" GPS. In the simplest example - if I choose to add a friend to "Find my Friends" on iPhone, both our locations are visible to each other via GPS when our phones are on.