Who should pay for improve-ments to rail services?
Should fare-paying passengers shoulder the huge cost of modernisation or should it be heavily subsidised through general tax funds?
That is the question underpinning the row over inflation-busting fare increases which have seen the cost of a rush hour return ticket between Edinburgh and Glasgow rise to £24.65.
Investment in services is the justification being offered for the biggest price rises in five years for Scottish commuters. In other words they are paying an even bigger chunk of the cost of improvements.
The Borders line and other projects have vastly improved the network in recent years, but the reliability of services remains patchy.
And, crucially, most passengers don’t rate it as value for money, with just 61 per cent of Scotrail passengers satisfied on that score.
As Edinburgh’s population grows, tempting commuters out of cars and on to public transport only becomes more important. Growing traffic congestion will not be enough on its own to encourage large numbers to switch. That takes a carrot as well as a stick.