I did a lot of travelling on trains with DC when they were young, and yes it - and other people - can be a nightmare.
If you know a route well, you can often adapt it a bit or get to know where the good seats are to help.
On my local trains (southern), the carriage with extra space for a wheelchair has doors marked red on the outside, so you can easily see which it is. That's a good one if you have a pushchair.
I would get on with my toddler still in the pushchair and sit at the seat which had space for the pushchair in front. Of course I would have moved if a wheelchair user got on but it was very rare.
I also amended a regular route so we would join
our connection a couple of stops earlier, at the terminus, specifically so we get a much better chance of a seat. It adds about 20 minutes to our 2 hour journey but it's worth it.
YANBU to expect people to be a bit more considerate, but sadly that just isn't the way it is around busy London trains.
Now the DC are older, I'm afraid I might be part of the problem! If we're travelling with a few of us together and would like a table, I've trained DD to weave (not push!) her way through the crowds to get ahead and save a table for us.
DP (a driver and not a regular train user if he can help it) stands there saying "after you" and I tell him you can't do that on trains, it's the law of the jungle out there!