Although it makes me sad to see homeless people about, I can understand how untenable this experience must have been. A tramp got on the underground in Vienna once and his odour gradually permeated the train and once it got to me I could hardly breathe. I was SO relieved that I reached my stop shortly afterwards, it was TERRIBLE! Therefore it can cause serious discomfort.

In this situation I would have said something to the driver to try to find a solution. I do not think I would have asked for a refund, however.