Historic steam trains - like the "Harry Potter" Jacobite Express in Scotland - could be forced off the tracks unless they fork out millions to comply with tough new health and safety laws, a railway boss has warned.
Heritage railway companies have been ordered to place bars over the drop-down windows of their 1950s-built Mark I former British Rail carriages to prevent excited passengers leaning out to take photographs while trains are moving.
The new safety measures were ordered by regulator the ORR (Office of Rail and Road) following the tragic death of a railway commuter in 2016 on the Gatwick Express.
The heritage trains must also fit central locking systems so that doors cannot be opened until trains have come to a standstill in the station.
The popularity of heritage railways running over the mainline network has soared in recent years.
Three hundred people paid more than £300 each on a Gala Dinner round trip aboard the Northern Belle's Pullman carriages which were hauled from Preston by Flying Scotsman on Saturday.
A spokesman for the Northern Belle, Britain's version of the iconic Orient Express, said: "Heritage railways like us have been given until 2023 to bring in these safety measures.
"This is going to cost millions - around £20,000 a carriage just to install central locking on each carriage. But you cannot put a price on safety and obviously we will comply as soon as possible.
"Bars over the drop-down windows will spoil the appearance of these Mark I carriages, though, some of which have been in use for up to 70 years without any other incident of this kind."
Regulator ORR (Office of Rail and Road) brought in the stringent new rules for all trains after Simon Brown, 24, was killed when his head hit a signal gantry in London after leaning out the window of the Gatwick Express in 2016.
GoviaThameslink Railway, who admitted breaking health and safety rules, were fined £1million at Southwark Crown Court last week.
After the hearing ORR Director of Safety Ian Prosser, who is also chief inspector of railways, said: "There are still some trains with these windows operating and we have written to operators instructing them to take action to prevent a similar tragedy happening again."