a colleague of mine at work was in the habit of leaving her brompton folded up by the door, whilst she took a seat. I would never do this – but she is a basically trusting sort
one day the inevitable happened and 2 young men in sportswear grabbed the bike and legged it off the train whilst it was stopped at a station
luckily she saw it happen, leapt off the train, ran up the platform, caught them up, grabbed the brompton from their sweaty clutches (I think she caught them by surprise by approaching from behind them). Not only that she also then leapt back onto the train just before it left the station.
she now stays with her bike whilst on the train