IT’s the flagship rail service between Scotland’s biggest cities.

Trains - many of them new and all of them electric - every 15 minutes shuttle between Glasgow and Edinburgh every 15 minutes

Or at least they should. This summer, in fact, nearly one in four ScotRail’s services on the core route was more than five minutes late or failed to run at all.

And that was before a calamitous evening on Saturday when the franchise was forced to cancel services just as the busiest day of the Edinburgh festivals was ending.

ScotRail yesterday apologised for weekend scenes described, reportedly, by one member of staff at Waverley Station as the worse they had experienced in a quarter of a century.

Tempers flared on overcrowded platforms and trains - dangerously so, said some eyewitnesses - as what are thought to be thousands of festival-goers and rugby fans tried to get home.

This was just a continuation of a summer from hell. The Public Performance Measure (PPM) for Edinburgh and Glasgow, or E&G as the route is called, was 76% in the four weeks between July 21 and August 17. That is the percentage of trains which ran within five minutes of their advertised timetable.

That number was down from 84% yearly on a rolling 12-month measure. It has not been easy on the route this summer, with problems often caused by things well out of the control of ScotRail, if not Network Rail, its state-owned track and infrastructure provider.

Winchburgh tunnel outside Edinburgh was flooded in mid-August, there was a signals failing at Haymarket and a problem with an overhead wire at Newbridge.

This weekend’s problems included a train breakdown, somebody trespassing on the line and drivings failing to show up for work or what ScotRail called “late call-offs”.

David Simpson, ScotRail’s Operations Manager, explained: “We had a plan for Saturday and that plan didn’t go well.

“We had more customers than we’ve seen in previous weekends in August which had worked quite well but I think with international rugby and the football being on the number of customers we had was excessive.

Speaking to BBC Radio Scotland, he added: “We also had a trespass incident at Curriehill, we had a train failure, and between them that meant that trains weren’t able to run as planned and that caused the crowd and congestion that we heard about through social media.”

But the dry PPM figures give a bigger picture. And that is that Scotland’s busiest and most important railway is also among its least reliable.

So the last four-weekly reporting period saw ScotRail as a whole secure a relatively respectable 88% figure on its PPM, despite Highland and E&G lines devastated by flooding. That was actually an improvement on the moving annual figure of just under 88%. ScotRail’s target is to get 92% of its trains within five minutes of advertised time.

So why is E&G worse than average, at 76%. And why does it persistently underperform? After all nearly three quarters of a billion has been spent electrifying the route.

The TSSA, one of the unions representing workers, pinned blame for Saturday’s scenes on management. It was “symptomatic” of how the company is being run it said. On Satursday, accross the network, the PPM was 80.7%. ScotRail was unable to say how many trains it had cancelled on Saturday or how many people’s travel plans had been affected.

A spokesman said: “We are now undertaking a full review of Saturday night’s events to find out exactly what went wrong, and learn any lessons to ensure we do better in future.”

The TSSA’s general secretary, Manuel Cortes, wants ministers, not ScotRail, to get to the bottom of poor performance on E&G this weekend. “No-one coming home from Scotland’s capital should expect to be squashed in to trains like cattle at 11:30pm,. Festival or no Festival.”