Comment by tialaramex

If you pass over a moderately complicated junction at low to moderate speed there's a good chance the signallers either can, or just by default will, derail you safely.

The phrase is "trap points". A correctly designed junction, where possible, will have points ("switches" if you're American) that default to leading to a dead end, with buffers and then nothing important. If you enter that track section without a signaller choosing where you should end up, that's what will happen.

The reason isn't for the implausible scenario of train theft, it's to reduce the consequence of errors. A wrong side signal defect allows a train into the junction without points being set - what happens? With trap points the train is derailed, probably at low speed, not directed into traffic. Or a driver misses a danger signal, same outcome. Expensive but probably everyone walks away.