Agonising back pain on train - is it enough to ask for a seat?

[Morelikeguidelines]

I suffer from a bad back and have often been to an osteopath which helps but is expensive.

If I stand still for long ish periods of time I get terrible pain - walking is fine just standing still a problem.

Is that a reason to ask for a train seat if full? I had not problem asking in pregnancy but feel like this might meet with angry responses as it is not visible.

I am currently sat on the train floor as nervous to ask and no one seems to find this odd!

Can I get one of those cards to say I need a seat?

I'd move if you asked.

Be ready for those that won't tho.
When I was 8mo preggo with a seat reservation, the man in my reserved seat refused to move saying I should've got there sooner.

The old lady opposite him, easily 80+ with a walking stick, offered me her seat, which I refused. He then added that I clearly didn't need it if I didn't take one when offered. The old lady told him he was an A$$hole.