ScotRail's punctuality over the last month was its worst for August/September since Abellio took over the franchise in 2015, figures revealed today.
It came as a ScotRail train broke down at Edinburgh Park, causing major disruption.
A total of 26 trains were cancelled and 14 others delayed by up to 1 hour 40 minutes on routes between Edinburgh, Glasgow and Dunblane.
The latest figures showed a total of 89.6 per cent of trains arrived on time (measured as within five minutes of schedule) in the four weeks to last Saturday.
That was 2 percentage points down on the same period last year - the standard industry comparison - and also lower than in the three years before that.
ScotRail said external factors outwith its control had had the biggest impact on performance for 18 months.
It said the most disruptive of these was a passenger pulling the emergency alarm on a train from Glasgow Central as football fans headed to the Scotland v Russia tie at Hampden on 6 September, which had a "severe impact on multiple routes".
Trains were also cancelled after a trespasser was spotted on the Forth Bridge three days earlier, while there will also an "incident requiring the emergency services at Falkirk Grahamston" on 22 August following reports of a man with a knife.
The punctuality figure was 1.6 per cent up on the previous four weeks.
However, ScotRail's underlying punctuality, the key measure that it is judged on by ministers, fell for the second month.
This performance figure for the last 12 months, or "moving annual average"(MAA), dropped by 0.2 points to 87.4 per cent.
That is more than 5 points below ScotRail's 92.5 per cent target, which it does not now expect to hit until 2022.
Scottish Conservatives transport spokesman Jamie Greene said: “Issues around punctuality and reliability have become a reoccurring theme for our rail network under the stewardship of this Scottish Government.
“Passengers have been hit by significant delays and disruptions over the past 12 months, and although some of these factors are outside of the operator’s control, it is worrying to see the MAA has fallen for two consecutive months.
“It is [transport secretary] Michael Matheson’s responsibility to get a grip of our rail network and ensure we do not see a repeat of the chaos passengers experienced this time last year.”
Scottish Labour said the poor performance had come despite a remedial plan in January year to improve punctuality.
Transport spokesman Colin Smyth said: “Despite two improvement plans and a remedial plan to improve punctuality, ScotRail’s performance is still falling.
"Customers deserve better than this shambolic service, and it is time for the SNP to stop protecting this failed franchise.”
ScotRail said there had been improvements since heavy rain caused flooding and landslips earlier in August.
It said: "The hard work of everyone across Scotland’s Railway [ScotRail and Network Rail Scotland], and the infrastructure improvements following the disruptive weather in August is having a positive impact.
"Additional work with key stakeholders and emergency services will look at how services can be recovered more robustly following external disruptions.
Alex Hynes, managing director of Scotland’s Railway, said: “It is encouraging to see an improvement for our customers, but even though nine out of ten trains running on time is good, we know there is more to do to keep the country moving.
“I understand and share their frustration when things don’t go to plan, and everyone across Scotland’s Railway is working round the clock to deliver the service our customers expect and deserve.”
ScotRail said the train breakdown involved the 10:35 Edinbugh-Helensburgh, due to a loss of power.
It said: "This resulted in services from Edinburgh heading west being subject to delays and alterations until around 12:30pm when the train was moved by an assisting train.
"To keep customers moving, we arranged for valid train tickets to be used on Edinburgh Trams and selected First Bus services. We also arranged some replacement buses.
"Services are now returning to normal, but taking time due to trains and crew out of position."
A ScotRail spokesperson said: “We’re sorry to customers who’ve been delayed due to one of our trains which broke down near Edinburgh Park.
“Both the driver and our control team worked tirelessly to fix the fault and allow services to get back on the move.
“Anyone who was been delayed by 30 minutes or more is entitled to claim money back through the Delay Repay guarantee on our website of mobile app.”