I wonder why....
I've taken basses on trains both long distance (Oxenholme to London) and local (Leeds to Lancaster), both in soft cases and hard cases. Nobody has ever batted an eyelid. As a previous poster has said, some people have enormous suitcases and people take those on the trains all the time. Try taking the Gatwick to East Croydon train and you can't move for massive suitcases, some of which I suspect could easily take an 8x 10 bass cab and the amplifier and leave space for a Marshall stack
I think the key is being sensible. Doing this at rush hour might get a raised eyebrow in London., but offer to move for more needy people and/or go off-peak and you'll be fine.
Not sure what the biggest package I've ever seen. I know my cousin, fresh from the USA, brought a Marshall 2 x 12 combo in Nottingham and took it on the train to Chesterfield. He may have actually put wheels on it and made it a new carriage. It took two of us to lift it off the train in Chesterfield as I recall.
Rob