This is an example (from a train in Japan). I circles the relevant section:
(Another problem is, I'm not sure whether to call those seats "chairs" or "benches.")
When the passengers entered, I retreated until my back hit the __
Maybe the railways have a specific word for them - but if you were to use it would anyone outside the industry recognise it for what it was?
You could call it a stanchion I suppose, but you could escape the dilemma altogether & just call it a metal bar, or pole or 'metal upright'... or for further distinction, 'the metal upright at the doorway, separating the entrance from the passenger compartment'... at the serious risk of becoming verbose ;)
Edit: for the other problem, use the same avoidance tactic - call them seats.
Another reason to avoid being too specific is the diversity of tube/subway trains across the world. That photo is of quite an old Japanese train, but compare it to a modern London tube - that's one heck of a lot of metalwork to hold onto, & no-one but the designer is ever going to distinguish between them.
Any reader is going to imagine their own local transportation, if they live in a city with an underground railway, or if not, one they saw on TV.