The empty Great Western Railway (GWR) "High Speed Train" was in flames at Exeter in Devon around 5am.
It among a fleet scheduled to be transferred to ScotRail to operate between Edinburgh/Glasgow and Aberdeen/Inverness from next year.
The fire came as ScotRail managing director Alex Hynes was due to unveil a new livery on another of the fleet at Aberdeen Station this morning.
It arrived in Scotland from GWR last week and has been repainted with the slogans "A New Era" and "We're building the best railway Scotland's ever had".
A GWR spokesman said: “We are currently investigating the cause of a fire in one of our train engines at our Exeter maintenance depot.
"As the train was out of service at the time, no customers or staff were affected.
"This caused delays of around 20 minutes to some of our services on Thursday morning."
Mr Hynes was due to be joined by Bill Reeve, rail franchise director of the Scottish Government's Transport Scotland agency to show off the first train.
It will be used for driver training over the next few months, but has still to be refurbished before entering passenger service.