PASSENGERS are finding it increasingly hard to find a seat on peak-period trains as the number of ScotRail services without enough spaces has shot up by 50% in five years.

More than 2600 trains a year were classed as not having enough carriages or being too small to meet the increased demand during the morning and afternoon peak in both 2010 and 2011, according to Government data released to The Herald following a Freedom of Information request.

The problem has grown worse since 2006, when 1621 services were found to be "shortformed", creating a headache for commuters travelling on an increasingly congested rail network.

Rail watchdog Passenger Focus expressed disappointment at the figures, which are being blamed on engineers failing to fix faulty trains promptly, and called on the Scottish Government to ensure there are adequate facilities to maintain and repair ScotRail's train fleet.

Robert Samson, the watchdog's manager for Scotland, said: "We're disappointed that the number of shortformed trains has gone up. One of passengers' priorities is getting a seat and if a train is shortformed it makes it all the more difficult and leads to overcrowding, particularly at the peak."

Mr Samson said shortforming problems had come about in part because of the extra services introduced by ScotRail, which operates more than 95% of passenger services in Scotland, since the start of the current franchise in 2004.

He added: "You have got to have adequate depot facilities to meet the needs of those services. The more trains you have, the more facilities you need to make sure they're properly maintained and can be put back into service promptly when there is a problem."

In order to meet the peaks of demand as people travel to and from jobs based in the city centre, ScotRail increases the size of its trains, typically by doubling the number of carriages from two to four or from three to six.

The increase in shortforming – when seat numbers on key services do not match Scottish Government requirements – in 2010 is thought to be due the severe winters that year and teething problems associated with the introduction of new electric trains operating on Inverclyde, Renfrewshire and Ayrshire routes.

However, the problems last year may have been exacerbated by maintenance issues with the 155 diesel trains operated by ScotRail, representing more than half of the overall train fleet. Data provided for the first three months of the 2012/13 financial year indicate a similar level of shortforming.

The majority of maintenance issues with diesels require a pit to access the underside of the train but there are currently only two depots with such facilities, at Edinburgh Haymarket and Inverness. Discussions have taken place about building an additional depot, but the plans have not been approved by the Scottish Government.

A spokeswoman for Transport Scotland, the Government agency responsible for rail services, defended its record of investment, which includes £24 million spent on a new depot at Shields Road in Glasgow and £5m investment in Haymarket depot to promote it as a centre of excellence for maintenance.

"The context of the issue of short formations has to be seen in terms of the significant increase in daily services, the impact of severe winter weather in 2008/09 and 2009/10 and the increase in ScotRail passenger numbers, which have risen by 30% since the start of the franchise," she said. "Improving performance in this area will be a key consideration of the next franchise agreement."

A ScotRail spokesman said any changes to contracted services were decided by Scottish Government ministers. He said: "We run more than 2300 services a day and spend a great deal of time planning their movements to meet anticipated demand.

"However, we have a finite supply of rolling stock and have to balance the needs of rail travellers across Scotland. Work continues with Transport Scotland and other stakeholders to support the increasing number of services we provide and to develop Scotland's railways."