You can also go really fast and intentionally derail the train, which is both a monetary and (depending on where you can get to) a security concern, or crash into another train on the same tracks (possibly a passenger train).
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.
edit: There's more!
https://media.ccc.de/v/32c3-7490-the_great_train_cyber_robbe... (and this one has English)