It's also not reasonable to expect an insurance company to pay out for the theft of an unguarded valuable item. It's no different to the fact that most won't pay out for a mobile phone left sitting visible on a car seat rather than in a locked glove box. Or indeed if you left one on the train table and went to the loo.
The upshot of this is probably simply that cycle carriage on a busy train is not an insurable risk; you choose to take the risk yourself, you run a second cheaper less attractive bicycle for such purposes, or you find a different way of getting where you are going.
I certainly have taken the risk of leaving a bag of clothes in the overhead and gone to the buffet/toilet, mitigating slightly by making sure I only go to those facilities when a station stop is not due, so if it goes missing I've at least got a small amount of time to find out where on the train it has gone missing to. But in the end I accept the risk of losing some not particularly valuable items, I would not expect this sort of loss to be insurable. My laptop etc go with me, and indeed are only insured if they are on my person (i.e. force or a threat thereof would be needed to part me from them) or they are in a private place (e.g. home) with the doors locked.
I'm sure an insurance contract could be arranged with a broker to cover a completely unattended bicycle on a train (just as you can probably get an insurance contract to cover a motor vehicle left parked with the keys in it). But because of the very high risk of theft, particularly if a valuable road bike, say, I'd expect that contract to be very, very expensive - possibly the full value of the bike per two to three years or even more (in the case of the car probably more than its value per year as theft is near certain). Most people don't want to pay that, so they have to take reasonable care to keep the cost low. I again maintain that it isn't reasonable care to have a valuable item out of your sight in a public place, whatever the reason for doing so.
Just about the only reason I could think of that might be acceptable would be being ordered by a Police Officer or member of railway staff to leave the bicycle unattended to the exclusion of all other options including leaving the train with it (e.g. if the train was on fire between stations and you were being evacuated from it), and even then that may not be covered if the reason for such orders was "force majeure" or terrorism.