Steam trains could be facing the end of the line amid plans to abolish traditional railway traffic light-style signals.
Network Rail plans to remove the lineside lights most train drivers currently use to guide their journey and expand in-cab digital signalling across the network instead, to cut upkeep costs and maximise capacity.
It means steam-powered engines will need to be fitted with these digital screens and software too if they are to continue running on the mainline and survive the network updates.
Trials are currently underway to determine if steam engines and historical diesels could be compatible with the so-called European Train Control System (ETCS) technology.
The experiments have seen Tornado, a steam locomotive built by train lovers in 2008 based on a 1940s design, fitted out with ETCS kit.
But if it does not work even in a more recent engine, older locomotives like the Flying Scotsman - built in 1923 and the oldest steam train still active on Britain's main network - face even less of a chance.
This could see the UK's some 500 steam engines - which contribute £600million a year to the economy - relegated to minor routes and preserved railways, the Telegraph reports.
Network Rail has said it will aim to make sure historic steam and diesel engines can still run - but a company source said the current trials offer no guarantee.
Steam trains could be facing the end of the line amid plans to abolish traditional railway traffic light-style signals. Pictured: File photo
The experiments have seen Tornado (pictured, with a digital screen top left), a steam locomotive built by train lovers in 2008 based on a 1940s design, fitted out with ETCS kit
Network Rail plans to remove the lineside lights most train drivers currently use to guide their journey and expand in-cab digital signalling (pictured, file photo) across the network instead
They explained: 'If we prove the concept with Tornado and it works properly, then we are in a better place to understand costs and feasibility for future deployment.
'But who will pay for that when there is a totally non-standard fleet of steam engines owned by a variety of different people is absolutely not certain.'
ETCS is already used on a handful of routes - but Network Rail plans to roll it out nationally, starting at the London end of the East Coast Main Line from King's Cross to Edinburgh.
The West Coast line from Euston to Glasgow and the Great Western route from Paddington are likely to be up next.
Instead of looking out the window for lineside signals, like traffic lights, ETCS sees train drivers observe a screen in front of them inside the cab.
It shows a target speed and gives them the remaining distance they have until they need to slow down or speed up.
Drivers can talk to signallers using an internal mobile phone system - and if they go too fast or run a virtual red light, the trains automatically brake, so are safer.
The technology also allows trains to be spaced out more efficiently, adding more journeys per hour.
Instead of looking out the window for lineside signals (pictured, file photo), like traffic lights, ETCS sees train drivers observe a screen in front of them inside the cab
Network Rail has said it will aim to make sure historic steam and diesel engines can still run - but a company source said the current trials offer no guarantee. Pictured: File photo
The main challenges with using the kit in steam and diesel engines are how hard it is to use a screen in an open, noisy, dirty and often wet cab and issues with braking, how the engine works and electrical supply. Pictured: File photo
The kitted-out Tornado - which was featured in Paddington 2 and Top Gear - was trialled overnight this week on the route from Shrewsbury, Shropshire, to Newtown in mid Wales, which is already ETCS-compatible.
The main challenges with using the kit in steam and diesel engines are how hard it is to use a screen in an open, noisy, dirty and often wet cab and issues with braking, how the engine works and electrical supply.
West Coast Railways - the network's biggest steam operator, also responsible for the popular 'Hogwarts Express' route - ran the test trains.
The firm has already faced challenges in recent years after the rail regulator prohibited its carriages from the 1950s, which train enthusiasts tend to prefer.
Rob Morland, an electrical engineer with Tornado's owners, the A1 Steam Trust, said a second steam turbo-generator, new alternator and third battery bank had to be added to the engine to facilitate the ETCS equipment.
Network Rail has said it is too early to comment on the results of the experiments.
A Deltic train, one of the most iconic diesels on the network, will be tested on the same line later in the year.
Amy Clouston, principal human factors specialist at engineering consultancy AtkinsRéalis, which designed the fitment of Tornado's systems, has previously told the Observer: 'We had to test whether or not you can even use a touchscreen while you're hurtling along at up to 75mph.
The kitted-out Tornado - which was featured in Paddington 2 (pictured) and Top Gear - was trialled overnight this week
In an iconic Top Gear episode of 2009 (pictured), the show's former frontman Jeremy Clarkson raced the Tornado train against fellow presenters Richard Hammond and James May from London to Edinburgh
'We've had to think about vibrations, lighting, noise, coal dust, water and steam and grubby fingers.'
Sheargued the project was vital to the future of the main line steam sector: 'Steam is part of the nation's engineering heritage so it would be such a shame if it were to only be contained to private, heritage railways.
'Having steam locomotives out there on the main line is a wonderful thing to see.'
In an iconic Top Gear episode of 2009, the show's former frontman Jeremy Clarkson raced the Tornado train against fellow presenters Richard Hammond and James May from London to Edinburgh.
The episode saw Mr Clarkson having to feed the boiler with a constant supply of coal, without which the whole train would grind to a halt.
