If the bus or train is crush loaded then no, people shouldn't be "thrown off". Though you mention buggies and luggage and such like. If you're in the wheelchair space with a buggy that you cannot or will not fold then you should get off. I used to use a pram that did not fold and it was a risk I took; I was always happy to get off if needed, though it was only ever required once.
But in reality how often is it the case that a train is crush loaded and nobody gets off? It happens at outer-suburban stations (e.g. Hornsey), but Clapham Junction sees a lot of people get off trains. Whilst it may be crush loaded when it arrives, and may be crush loaded when it leaves, it will be different passengers on board. A wheelchair user should have priority boarding against able-bodied passengers. That much is blatantly obvious. People in the wheelchair space should move; that is equally obvious.
I don't see many buses that are filled to the plated capacity, and you'll note that the plated capacity usually only substitutes two standees for a wheelchair user. What's more common is that people are sitting in the tip up seats in the wheelchair area, rather than standing, and grumble at having to move. Tough luck. It's why I prefer London buses for not using tip up seats in wheelchair areas, people are much less likely to be in it.
Trog said:
You would then probably be responsible for looking after said bags, and returning them to their owners. As to deprive a person of their property with the intention of not returning it is almost the definition of theft.
That'll be why Network Rail make it clear that if you leave your property somewhere you shouldn't, that it'll be "removed and may be destroyed by the security services".
The example I have in mind is my last trip on EMT. Some idiot had dumped a load of bags in the wheelchair area in first class. Rather than move the bags off the train at Leicester when a wheelchair user got on, they left the wheelchair user in the vestibule. That was wrong. The excellent first class host sorted it, eventually, but those bags should have been moved off the train at Leicester, with a message at London saying the owner can collect them at their expense and convenience. Without the first class host the wheelchair user wouldn't have been able to get to the space they'd reserved.
"Bags left here will be removed" would cover it, just like my employer removes bikes that are chained to the railings out the front.
I see it all the time on EastCoast, with the idiots travelling with a suitcase the size of a small car dumping it in the wheelchair area. Sorry, but those bags should be removed from the train.