Managing director Alex Hynes revealed to MSPs the first of its refurbished InterCity trains on the Aberdeen-Edinburgh would not enter service until at least July, rather than May as planned.
He also said a planned ten-minute journey time cut to 42 minutes on the main Edinburgh-Glasgow line would not be fully introduced until next year.
Only some trains would run at the faster time in December as scheduled.
Mr Hynes told the Scottish Parliament's rural economy and connectivity committee the troubled brand new Hitachi electric fleet for the line were still months away while new windscreens are fitted and software problems solved.
The setbacks came as new figures showed a further fall in the key punctuality measure on which ScotRail is judged.
A total of 89.3 per cent of trains arrived at their destinations within five minutes of schedule in the year April, down 0.2 percentage points on the previous month and nearly 2 points down on August.
However, 92 per cent of trains hit that target during April, up 1.1 per cent on March and the highest since September.
There was further misery for passengers today, with another electrical fault near Rutherglen causing widespread disruption to cross-city trains in Glasgow, which has the biggest network outside London.
The latest of a series of problems followed Glasgow Central - Scotland's busiest station - being shut on Saturday night by overhead line damage, and signalling faults in Glasgow yesterday.
Mr Hynes said the refurbishment of 40-year-old InterCity trains which have come from Great Western Railway had been a "challenge", but he wanted to get the first completed one to Scotland from the works in Doncaster as soon as possible.
He said they would transform travel between Edinburgh, Glasgow, Aberdeen and Inverness with more carriages, more legroom and tables, and seats more aligned with windows.
He said the performance problems were partially because ScotRail had dramatically cut the number of trains which skipped stops during disruption to minimise knock-on delays to other trains.
Mr Hynes said the 70 per cent reduction in skip stopping had resulted in a corresponding fall in passenger complaints, but it meant the rail network took longer to return to normal.
He said the shortage of trains caused by the Hitachi delays, and others coming to end of their leases, would be more than offset by ten new trains which will go into service in July on the main Edinburgh-Glasgow line.
New windscreens are being fitted to the Hitachi trains to remove a "slight double imaging at night" which drivers said caused problems seeing signals clearly.
Mr Hynes said: "An alternative design is being fitted to the trains this week.
"Initial indications are the windscreen is much better than its predecessor."
Mr Hynes said fines levied on ScotRail for failing to meet quality standards, such as on CCTV, litter and train seats, would be spent on improvements such as waiting room upgrades on the main Edinburgh-Glasgow line and information screens on the Inverness-Thurso/Wick route.
A total of Â£1.6 million was charged in the last three months of last year under a regime which Mr Hynes said was the toughest in the UK.