The growth in the use of the railways is a success story on many levels. Rail travel is more efficient, and environmentally friendly than many of the alternatives. It is an aid to reducing congestion in our town centres. And it is increasingly popular - with passenger journeys up by 35 per cent on the Scotrail network, over the past decade.

But the capacity has not grown along with the demand. Provision of rolling stock remains a stubborn problem, with the number of seats growing by just 15 per cent over the same period, and capacity reduced even further if you take a reduction in standing capacity into account.

Remarkably there has only been one additional seat provided for every 3,300 additional passenger journeys made on the network.

This creates frustration and anger among passengers - just ask the commuters who may pay thousands of pounds to travel regularly between Scotland's cities, yet who may not even be guaranteed a seat. Or the passengers on the new Borders Railway facing standing room only or even cancellations as trains run with only two carriages.

Running trains underoccupied or near empty makes no sense and passengers understand that. But there are routes which are routinely overcrowded, where more trains are plainly needed.

Abellio, the current holders of the ScotRail franchise, have pledged to invest in leasing additional rolling stock, and the company must deliver on the promises made when taking up the contract. But improvements appear painfully slow. Brand new Hitachi trains are to replace outdated models, but only from 2017, two and a half years into Abellio's current seven year contract.

Additional long distance high speed trains serving Edinburgh, Glasgow, Aberdeen and Inverness are not expected until December 2018.

This is not a new problem. Abellio's predecessors frequently wrestled with rolling stock shortages and appeared unable to respond quickly.

To the outsider this is baffling. If demand is there, provision should follow. The internal market of the railways is obviously failing when there are simply not enough trains in the UK and this situation endures for years.

The reason may be because it is more economic for train companies to risk overcrowding with less rolling stock, than to provide more space and risk underoccupancy. Or it may be that the companies which lease trains to providers such as Abellio have a perverse incentive to keep them in short supply and prices high. Perhaps the problem is the relatively short term nature of franchising, with companies unwilling invest significantly in services which they may subsequently lose. It is likely that a combination of these factors is to blame.

But the problems with rolling stock are merely a microcosm of wider rail privatisation with the benefits of competition for passengers still obscure after 21 years.

New rolling stock is welcome, when it comes. But the public will continue to have concerns while our railways are significantly more expensive than those elsewhere in Europe, without offering a correspondingly high service.