It's not entirely untrue through. Once you deduct the cost of railway
improvements, and new lines like Crossrail, HS2 etc., the state pays "just" £200-300m a year towards the on-going costs of running and maintaining the railway. Not bad considering the number of people it moves, the environmental and social benefits and the financial benefits it unlocks in a way other means of transport can't.
The decision to spend the better part of £10bn a year into the railways to upgrade and improve them is a political one. If push came to shove you could continue to run the railway on a shoestring budget of the aforementioned £200-300m a year. Obviously it would be at the cost of ever increasing overcrowding but it is theoretically possible. This is considerably less than the
average of £1.5bn a year that BR required in its latter years (using inflation-adjusted sums).
There are many valid criticisms to make against the privatised railway. The fragmentation of the industry has undoubtedly led to more inefficiencies than private "efficiency" has gains. But skyrocketing passenger numbers and real-terms fare increases have, despite this, allowed a real-terms reduction in underlying subsidy.
If the political aim of the railways were to make them pay, it would probably be possible, just about, by increasing fares even more in real terms, thus reducing overcrowding and extracting more money from the remainder who have the ability and willingness to pay. I think that most political parties recognise this is not politically expedient.