Network Rail is planning a modernisation programme which could see the number of signal workers drastically cut.
"There have been discussions this week over future signalling arrangements on the rail network," said RMT general secretary Bob Crow.
"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."
The number of signal boxes has been reduced from 10,000 at the beginning of the 20th century to around 800 today, and Network Rail wants to introduce a new system which could see the number cut to just 14.
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."