Sadly, yes, this happened to me once on a Scotrail train from Edinburgh to Glasgow. Nobody said what was going on, but the sudden stop, followed by the driver rushing through the train towards the toilet, made it pretty obvious. We were sat there for about 2 hours before we moved to the next station, and had to change trains.

The sad thing, for me, is that this journey was following a phonecall to say that my Grandmother was critically ill, so I was headed up to Scotland as quickly as possible. I jumped on the first train North, instead of the later direct one. If I'd waited, I'd have got there in time. Sadly, I didn't. Not a good day.