1. Literally, of a train, to derail from the track and lose control. Due to a technical issue, the train wasn't able to slow down ahead of the turn and ended up jumping the rails because of its speed.

2. By extension, to veer off in very unexpected directions; to lose or change focus in surprising or bizarre ways. The long-running drama has by this point jumped the rails so completely that it would be foolish to try and summarize it for the uninitiated. The manager's speech really jumped the rails about halfway through, shifting into a weird commentary on the nature of corporate America.