We can't see how this works Young Owen, sorry, just can't see it

Owen Jones tells us several things about the current rail system:

If you live in London and urgently needed to make it to Leeds today, you would have to part with at least £98.70. A British Airways flight to Paris booked today, on the other hand, will leave you £62 worse off. The cheapest train to Edinburgh today costs £128.20. There are cheaper flights available to Madrid (£88), Berlin (£90) and Rome (£112). The proportion of British commuters’ pay packets spent on train tickets is up to issix times higher than their European equivalents.
As a 2013 report found, the state shells out billions more on public subsidies for railways than it did in the days of British Rail. And after inflation, overall public spending on the railways was an astonishing six times higher in 2013 than after privatisation in 1996.

Train tickets cost more in the UK than in many other countries. Quite true. That's because those travelling pay rather more of the cost of their travel than do people in other countries. That is, the general taxpayer pays rather less of the bill, subsidising the travel of other people. Which, we are really pretty sure, is the right way to be doing it. Those who get the service should be the people paying for it.

However, we also get told this:

The case for publicly run rail is popular, and for good reason. It would cost nothing to bring rail franchises into public ownership as they expire, and they could prove to be cash cows for the Treasury.

It's possible that the railways are, will be or could be profitable whoever owns them. It's possible that they are, will be or could be in need of large subsidy as they are not profitable, whoever owns them. But this is the bit we cannot understand. How can we describe something which requires large subsidy as being a cash cow? Doesn't compute, does it?