Signal Failure
You will often hear announcements on the underground about severe delays to particular lines, due to "signal failure". Based on the frequency of such announcements, you may wonder why the signalling equipment is so unreliable as to keep failing.
In fact, the term "signal failure" merely means that a train is stuck at a red light. The cause of this is almost certainly something legitimate, such as a queue of trains heading through a bottleneck (Acton Town Station, Earl's Court Station and Camden Town Station are examples of such bottlenecks).
A signal failure could be caused by something more serious, such as a points failure, something obstructing the track, or a [passenger action]?, though this cause will usually be announced rather than the bland "signal failure"