Yesterday, I removed my shoes on a British train for the first time. I have removed my shoes on Japanese trains and on aeroplanes (I am amazed that some people keep their shoes on during flights!). It may not be very British, but I would rather travel in comfort.
I noticed a woman had put her shoes up on a seat. I made sure I put my shoeless feet up on the seat next to me to set a proper example. I remember a train many years ago that had a sign not to put shoes on the seats, but I have never seen one since. Neither the passengers or the operators seem to care.
Of course, I am sure some old-fashioned people disapprove of others taking off their shoes on trains. However, if I can get away with taking my shoes off on planes and I have to take my shoes off at airport security, why should I not remove them to make train journeys more comfortable? This is the modern world.