The revelation comes amid concern within the rail industry at ScotRail’s lack of promotion for the trips, which start today with the “Highland Rambler” between Stirling and Inverness.
The first steam excursions by Scotland’s main rail operator for 30 years ago will also go to Brora and Carlisle over the next week.
A separate set of steam trips will run on the Borders Railway from September, for which tickets have yet to go on sale.
The excursions are part of new ScotRail operator Abellio’s plans to encourage more visitors to travel by rail.
These include marketing rural lines as “Great Scenic Rail Journeys” and upgrading carriages to become “scenic trains”, with seats aligned with windows and local catering and information.
Jeff Hoogesteger, the then Abellio Group chief executive, told The Scotsman last October: “We are aware the opportunity to experience rail travel by steam train is popular with people in Scotland and with tourists.
“With effective marketing, we believe the steam train experience will increase visitor opportunities.”
Tickets for the trips have been on sale on the ScotRail website since Abellio took over in April at £114 for first class and £59 for standard class. However, this week, seats have been offered on the discount website Itison for £55 and £29 respectively.
The trains will be hauled by Tornado, a locomotive built in 2008 based on the A1 class engines which ran on the east coast main line in the 1940s.
It has made several trips to Scotland, including racing a car between London and Edinburgh on BBC2’s Top Gear.
Passengers will travel in former British Rail carriages from the 1960s, operated by Bo’ness-based Scottish Railway Preservation Society Railtours.
One industry source told The Scotsman: “ScotRail has not done much to promote the trips and they had not sold as many tickets as they had hoped.
“They don’t have the experience – it’s not the sort of thing they have run.”
He also said the Borders trips would have been better run in July or August to coincide with the Edinburgh Festival, but the line will not be ready until September.
Another rail expert said that, despite the popular appeal of steam trains, ScotRail needed to have done more to publicise the trips. He said: “Most current steam trips in Scotland are tours from England, so they are effectively trying to establish a new market.”
It is understood that ScotRail had been reluctant to actively promote the journeys until it completed negotiations with their new operator.
This came after it axed original operator West Coast Railways when the firm was temporarily banned from the rail network for running through a red light on to a main line in England.
ScotRail said it began promoting the journeys as soon as a deal was struck with new operator DBS Rail UK.
Head of marketing Nesta Gilliland said: “Ticket sales are healthy, particularly for the weekend, which are almost sold out.
“We’ve been working with VisitScotland and, thanks to a promotional offer with Itison, we are delighted to confirm we will be donating all income from this channel to Breast Cancer Care.
“In common with airlines and other transport operators, we see ticket promotions as an ideal way of encouraging leisure and off peak travel.”