Image copyright ScotRail Image caption The first of the upgraded trains in Edinburgh when it arrived in Scotland

The first of a fleet of upgraded high-speed trains (HSTs) is to enter service in Aberdeen next week, it has been announced.

Rail operator ScotRail said the first of the 26 newly-refurbished Inter7City trains would run from Aberdeen to Edinburgh via Dundee from Monday.

The diesel trains date back to the 1970s but they have all been renovated, at an estimated total cost of £54m.

The first of the refurbished trains had been supposed to be introduced in May.

But ScotRail admitted the refurbishment project had been "a challenge".

The Intercity 125 Class 43 HSTs (high-speed trains) will operate on the lines from Aberdeen and Inverness to Glasgow and Edinburgh, which take in Dundee, Perth and Stirling.

The rail operator's boss Alex Hynes is reported to have told an SNP conference fringe meeting earlier this week that "people don't care how old their trains are".

Image copyright Scotrail

He said the upgraded trains would offer a third more capacity on some trains, quicker travel and a more comfortable journey than the old Class 170 diesel multiple units (DMUs).

The HST was the mainstay of British Rail's inter-city service and is still the world's fastest diesel train.

However, ScotRail HSTs will have a maximum speed of 100 mph.

The operator said the first of the trains would help deliver Scotrail's improved timetable from 9 December.

ScotRail hopes the other trains will be phased in on the Inverness and Aberdeen lines over the next year.

They are undoubtedly one of the nations favourite trains. Bruce Williamson, RailFuture

The Class 43 HST trains were first introduced in the late seventies and have millions of miles on the clock.

The refurbished trains have been released by Great Western Railway (GWR), which operated them on long distance journeys between London and the south west of England.

The carriages have been refurbished in Doncaster, South Yorkshire, and the power cars in Loughborough.

They had been previously refurbished just over a decade ago.

The trains will be in sets made up of two Class 43 power cars, one at each end of four or five Mark 3 carriages.

Image copyright Scotrail

Abellio Scotrail has leased 26 sets for 12 years, spending an estimated £54m on refurbishments.

Overall, 17 upgraded five-carriage trains and nine four-carriage trains are scheduled to be used on lines across Scotland by late 2019.

ScotRail said the trains would eventually make journey times shorter and have more seats and luggage space available for passengers.

They will also offer an enhanced food and drink service, with a new cafe area on board in addition to the usual trolley selection.

Bruce Williamson, a spokesman for the campaign group RailFuture, told the BBC's Good Morning Scotland people should not be deterred by the fact the trains are very old.

He said: "They are really well-built trains and passengers love them. They are undoubtedly one of the nation's favourite trains.

"They will have more seats, they'll be faster and, having been refurbished, they'll have lovely interiors."

He added: "The ideal, I suppose, would be to have brand new electric trains but hopefully the introduction of new 125s to Aberdeen and Inverness will result in an increase in passenger numbers."

Mr Hynes, ScotRail Alliance's managing director, said: "This is another major milestone in our plan to build the best railway Scotland has ever had.

"Our Inter7City service will provide more seats, better services, and more comfortable journeys for our customers.

"The investment we are making in these iconic trains is a clear sign of the ScotRail Alliance's commitment to building a bigger and better railway for the whole of Scotland."

Transport Minister Michael Matheson said: "The entry into service of the first fully upgraded high-speed train is to be welcomed and I look forward to seeing more follow in the months ahead.

"The boost in seating provision will help support the overall 23% increase in capacity being delivered during 2019 as rail continues to grow in popularity across Scotland."

Do the refurbished carriages make the grade?

Image caption David Henderson took a ride on one of the refurbished carriages

BBC Scotland's transport correspondent David Henderson hopped on board the upgraded high-speed trains (HSTs). Here he answers what are likely to be key passenger questions......

What is the wifi like?

There is WiFi throughout the train, (but on this occasion I didn't have time to test it). There are charging points for mobile phones and other electrical devices - but only the double seats with a table seem to have charging points next to them.

Are the toilets ones you would use?

The toilets seem pretty smart and very roomy with push button doors to open, close and lock. Also, the hand basin has hand sensors to turn on the water and hand dryer.

Are the seats comfy?

The seats are firm but comfortable with a high headrest and smart blue and white ScotRail pattern fabric. The first class seats were luxurious, big leather affairs.

Any difference to luggage racks, compartments?

The carriages have standard luggage racks above the seats as well as a luggage area at each end of each carriage for suitcases - fairly standard. On balance, there appears to be good space for long distance travellers to store their bags.

What's the deal with refreshments?

As well as the standard trolley service a small on-board cafe serves hot and cold drinks, snacks and sandwiches.