Steam trains have been banned in England and Wales over fears they could start wildfires during the heatwave.
Network Rail said it had asked charter operators to temporarily replace the engines with diesel trains, to avoid sparks and burning cinders.
It operates a traffic light system for steam to reduce the risk of fires, ranging from green (no restrictions) to black (total ban).
England is enduring its second heatwave in a month, causing dry vegetation and tinderbox conditions, with no measurable rain for 15 days – the longest dry spell since June 1996.
In Scotland, a large wildfire has been burning in the East Highlands for more than three days, prompting the evacuation of residents.
However, Network Rail said the ban, which came into force south of the border on Monday, did not yet apply in Scotland.
On Saturday, an engine used on “Harry Potter” tourist services was linked to a fire near Winsford, Cheshire, that led to the cancellation of 72 passenger trains.
The cause of the fire is still being investigated, but the West Coast Railways (WCR) engine is believed to have emitted sparks during the incident.
The locomotive is used on WCR’s “Jacobite” service, which has become known as the Harry Potter train because it passes over Glenfinnan Viaduct, used in the films of JK Rowling’s novels.
A Network Rail spokesman said: “The current long periods of hot dry weather have increased the risk of wildfires across the country.
“As a result, we have asked charter operators to replace steam engines with diesels in line with our steam charter fire risk policy.”
Passengers preparing to board the Spirit of Summer Lunch steam train experience from London Paddington were told that the service would be hauled by a diesel engine.
A message to customers two days before the £180-a-head trip said that a steam locomotive could not be used because of the ban.
“Our trip will still run as scheduled on Thursday, using the same beautifully restored vintage carriages and offering the same high standard of on-board service,” it said.
“In place of steam, the train will be hauled by a vintage diesel locomotive.”
The Sun reported that the Northern Belle, which costs £595 per head, had also had to switch its locomotives.
A spokesman for the luxury service told the newspaper: “Obviously this is a big disappointment for both us and many of our passengers.
“But there is a very high fire risk after all the recent hot weather so we are using a heritage diesel locomotive instead.”
The company is refunding £50 to passengers and giving them a complimentary cocktail onboard.