According to a scientist who advises the government, a high tech signalling system that will eventually control all of Britain's trains could potentially be hacked to cause a serious crash.
Professor David Stupples said plans to replace ageing signal lights with new computers could leave the rail network exposed to cyber attacks.
Currently, UK tests of the European Rail Traffic Management system are in progress and Network Rail, which is in charge of the upgrade, acknowledges the threat.
A spokesman for Network Rail said: "We know that the risk [of a cyber-attack] will increase as we continue to roll out digital technology across the network.
"We work closely with government, the security services, our partners and suppliers in the rail industry and external cybersecurity specialists to understand the threat to our systems and make sure we have the right controls in place."
Once the system is up and running, computers will dictate critical safety information, including how fast the trains will go and how long they will take to stop.
The system is said to take command of trains on some of the UK's busy intercity routes by the 2020s.
The system is being used in other parts of the world and there are currently no reported cases of it being affected by cyber attacks.
Professor Stupples said if someone hacked into the system however it could cause a "nasty accident" or "major disruption".
He said: "It's the clever malware [malicious software] that actually alters the way the train will respond.
"So, it will perhaps tell the system the train is slowing down, when it's speeding up. Governments aren't complacent.
"Certain ministers know this is absolutely possible and they are worried about it. Safeguards are going in, in secret, but it's always possible to get around them.
"We keep security arrangements under constant review to take account of the threat and any new challenges we face."
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