THE sudden closure of the Forth Road Bridge almost two weeks ago has exposed historic frailties in Scotland's infrastructure that has caused headaches for commuters, ministers and transport chiefs.

The repairs and maintenance record of the bridge itself has inevitably descended into a political war of words which the parliamentary recess, starting today, is unlikely to quiet.

While rival parties may feel all their Christmases have come at once, Derek Mackay has been delivered a proverbial lump of coal in the form of a 2cm-long crack in a steel beam.

The questions of how, when and why it appeared will rumble on into the New Year, perhaps only settled by a public inquiry.

But if any area of transport has been caught like a deer in the headlights by the disruption, it has been Scotland's rail sector.

As with the opening of the Borders Railway three months ago, ScotRail was again lambasted by commuters over cramped carriages as the finite supply of rolling stock was stretched even thinner to cope with increased demand on the Edinburgh-Fife routes.

A handful of services elsewhere in the network were temporarily cancelled while others saw their frequency slashed as carriages were relocated from the west of Scotland to the east.

Even so, some Edinburgh-Fife routes were still so packed that would-be passengers were left stranded on the platform.

ScotRail has borrowed additional rolling stock from rail companies in England, but doing so will likely come with a hefty price tag such is the demand for extra trains across the whole of the UK.

The rail industry is simultaneously a victim of its own success, and of decades of underfunding. Passenger numbers have boomed while the delivery of brand new stock has lagged far behind.

The shortage of rolling stock in the ScotRail fleet is not really the fault of ScotRail bosses - though the angry outpourings on social media might suggest otherwise.

ScotRail run the service, but investment in extra trains is a matter for government and train operating companies. In ScotRail's case, the Scottish Government and incumbent operator Abellio - and prior to them, First Group and National Express, stretching all the way back to the dawn of the privatised era in March 1997.

Somewhere along those two decades, the ball has been dropped - though it is not a situation unique to Scotland.

Buying brand new trains is hugely expensive. Running half-empty trains is also extremely costly.

For a long time, ministers and operators have hedged their bets as passenger numbers grew. That gamble is coming to a head now.

While annual passenger journeys have surged 35 per cent in the past decade alone, capacity - both seating and standing - has increased by less than 10 per cent. And that is only if every single carriage is in use.

As a result, ScotRail's busiest services are carrying more than twice as many passengers as there are seats.

Yes, more trains are on the way under the Abellio franchise from December 2017, but that is cold comfort for commuters who have seen their morning train scrapped to cope with a road bridge defect on the opposite side of Scotland.