I had pelvic problems in pregnancy so was on crutches for the last few weeks. I stood on the train into London many times, while people pretended not to see me. I didn't like to ask, as I know other people can have their own reasons for needing to sit, but it made me very upset!
One time, a man kindly stood up for me on the tube, and someone else pushed in and took the seat! He turned his headphones up and didn't look up from his book the whole journey. I was gobsmacked.