Except that's the bare minimum of what they do, they're also fairly critical to the safe running of the trains, have to be trained in safety on the rail network, be able to assist in an emergency, act to some degree as security and a 101 other things.Walking the aisle checking tickets for £33k a year? There are people working warehouse jobs and doing care work for £16-£20k who would bite your hand off for that.
You might as well say that a flight attendant is just a trolly pusher, and ignore their importance in the safe operation of aircraft, and the safety of passengers when things go wrong (both flight attendants and "ticket inspectors" can/do have to be able to make the decision to evacuate their respective modes of transport and get the passengers to safety in what can be extremely dangerous situations).