There's a device that derails trains that enter unauthorized areas!
That's where a derailer comes in. As it's name suggests, a derailer derails the equipment that rolls over it.
As you might imagine, this can cause a lot of damage to the train that runs over the derailer. However, this damage is nothing compared to the destruction, injury or death that could occur if the train were allowed to proceed into the unauthorized area.
Derailers are installed where side tracks meet main lines, at junctions or other crossings, at an area where crews are working on a line, and anywhere approaching a drawbridge, dead end or other hazard.
