The Rail, Maritime and Transport (RMT) union has threatened to take travel disruption across the country, after it set a deadline to two train companies for assurances on the role of the guard.
It has told both Merseyrail and Arriva Rail North, which trades as Northern, that it wants assurances over the "safety critical role of the guard" by 26 January or else it will be "in dispute with" the companies.
Merseyrail plans to introduce a new fleet of 52 the so-called driver-only-operated (DOO) trains in Merseyside from 2020, and the RMT's general secretary Mick Cash, said the union's position was "perfectly clear".
"We will not agree to any extensions of DOO and will fight to retain the safety critical role of the guard and keep a guard on the train."
He said Merseyrail had failed to provide the union with "cast-iron" assurances that new trains "will have a second safety-critical crew member on board".
In a statement Merseyrail said it was "surprised and disappointed" that the RMT had threatened a dispute, as there had been "no substantive face-to-face discussions" between the two organisations on the role of driver-controlled operations.
It said: "The new trains will be safer than the ones they replace - and there will be staff on board focusing on customer duties."
The trains will have drivers controlling the doors, though there will be staff on board to help advise passengers.
"None of today's permanent guards or guard managers will be forced to leave Merseyrail's employment," the company added.
Arriva Rail North has also ordered new trains.
A spokesperson for the company said: “Northern began the new franchise in April 2016 and is committed to an ambitious programme to modernise rail travel for our customers. The improvements will maintain high safety standards, deliver better journeys and improve customer service across the network."
They added that it was "really important to us that our colleagues and their representatives are involved in discussing how to bring these improvements to life".
"This is not the time to talk about a potential dispute, we want to sit down and work positively with the trade unions," the spokesperson said.
It comes after months of disruption has impacted the Southern rail network over a dispute centred around the role of the guard on trains too.
Industrial action is being held by train drivers' union Aslef this week, with further dates called for 24, 25 and 27 January.
The RMT also announced today that it will be balloting Tube workers for more strikes after a 24-hour walkout caused havoc in the capital earlier this week.
It has started a separate ballot of Arriva Rail North staff in a pay dispute, saying a pay offer didn't match benchmarks for deals the RMT had set elsewhere in the industry. That will close on 25 January.
Northern's spokesperson said it had offered over 5,000 employees "a guaranteed, above inflation pay rise over the next three to four years".