Network Rail has started consulting on plans to drastically reduce the number of signal workers and signal boxes as part of a modernisation programme aimed at saving £250 million a year and improving punctuality.
The Rail Maritime and Transport (RMT) union said the plans could see the number of signallers cut from 6,000 to 2,000 over the next 30 years.
There were 10,000 signal boxes at the start of the 20th century but that number has been reduced to around 800 today, but NR wants to introduce a new system, widely used in the United States and Europe, which could see the number of locations cut to just 16.
An NR spokesman said: "Network Rail is in the early stages of a proposal that could accelerate its signalling modernisation programme, delivering significant benefits in terms of more punctual services, more flexible services, better passenger information and savings approaching £250 million per year.
"It's a long term strategy, phased over 15 to 30 years, and we have started to discuss its implications with both the trade unions and our people."
The RMT made it clear it will not accept any compulsory redundancies and will be seeking a shorter working week, increased annual leave and voluntary retirement at 55.
General Secretary Bob Crow said: "There have been discussions this week over future signalling arrangements on the rail network. Those discussions are at an early stage.
"RMT wants to make it absolutely clear that we will not agree to anything that compromises the job security, safety or standards of living of our members.
"We are not opposed to new technology but we are clear that any changes that may arise should be accommodated through a shorter working week, additional annual leave and the right to retire at 55 on full pension entitlement."
* Services between Portsmouth and London Waterloo were delayed during the rush hour this morning because of signalling problems.