Mick Lynch has warned industrial action across the rail network will continue beyond May unless the RMT receives a reasonable offer from the UK Government.
Members of the union are striking at Network Rail and 14 different rail operators this week, with ScotRail services hugely disrupted.
Speaking from a picket line at Euston station in London, Lynch said that the strikes "are likely to go ahead if there’s no offer that we can work on.
“We would like to get into a situation where we’re negotiating constantly with the companies and where we didn’t have to have strike action, and then work up a settlement that our members could vote on and accept.
“But if we don’t get that there will have to be more action, and we’ve got a mandate that runs through to May this year, and if we have to go further, that’s what we’ll need to do. We don’t want that, though.”
Network Rail staff are striking on Tuesday, Wednesday, Friday and Saturday.
Services will also be severely affected on Thursday due to the challenges of restarting a timetable with just a few hours between strikes.
A ScotRail spokesperson said there would be only a very limited service across 12 routes in the central belt, Fife and the Borders.
Lynch added that thousands of workers would have lost their jobs if the RMT had accepted the latest offer from the rail companies. He also said that this would have made the railways more dangerous.
He said: “The one on the train-operating companies we could never accept, because that would mean thousands of our members and guards would lose their jobs, that would mean a less safe railway and it would also have meant we’d have to accept a pay offer that is way below last year’s inflation, and this year’s inflation, 2023."
Train drivers in England will also walk out on Thursday, which could affect cross-border services.
Trains that do run will start later and finish much earlier than usual, with services typically running between 7.30am and 6.30pm on the day of the strike.
The train drivers’ strike on January 5 will affect 15 operators and will result in even fewer services running, with some companies operating "very significantly reduced" timetables.
The RMT also has an overtime ban in place at 14 train operating companies until Monday that will continue to affect the level of cancellations and the punctuality of some services.
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