A month of misery for hundreds of thousands of rail commuters begins on Monday when RMT members of South Western Railway walk out on the first of 27 days of strike action.

Hundreds of guards and a small number of drivers will not clock on for work again this Christmas, apart from on election day on 12 December, in a bitter row over the role and responsibilities of onboard train crew. The train firm was among others to be hit by similar strike action last year over the role of guards.

Only about half of normal weekday services will run on SWR, the UK’s second biggest train operator with about 600,000 passenger trips a day from Surrey, Hampshire and beyond into London Waterloo, the country’s busiest station.

Services will be cut entirely on some routes, with replacement buses in parts, and passengers diverted on to other rail operators’ trains. Commuters have been warned to expect extremely overcrowded peak services and even queues to enter stations.

Two days of talks at Acas last week ended acrimoniously, with the RMT accusing the train operator of refusing to make concessions to avert the strikes, while SWR said the union was “repeatedly moving the goalposts” and “intent on striking no matter what”.

New trains introduced on SWR allow for driver-only or driver-controlled operation. While the train company has pledged to retain a second crew member aboard, the RMT has sought further assurances, including changes to procedures when boarding, that would keep safety-trained guards integral to the operation, even with drivers controlling the doors.

The union’s general secretary, Mick Cash, said last week that SWR’s plans could “reduce the second person on the train to little more than a passenger in the longer term, which would give the company the option of axing them altogether at some point down the line”.

Cash urged the company to “grab with both hands” a solution to avoid disrupting passengers.

SWR, however, said it had done “everything we can and more to meet the RMT’s outdated demands”, including a safety critical role for guards. But it said the proposals from the union would compromise the improved performance, safety and customer service that the new fleet of trains would deliver.

The RMT claimed that SWR could benefit from up to £86m in reduced payments to government because of clauses in its franchise contract over lost revenue from industrial action – effectively giving it no incentive to reach agreement and avoid a strike.

SWR refused to comment, saying any such terms were a matter of commercial confidentiality.

Both sides say they remain available for negotiations but there appeared little prospect of agreement last week.

The long-running dispute over guards and driver-controlled operation of trains first prompted prolonged industrial action at Southern. It has since spread to networks including Northern, Merseyrail and most recently West Midlands.

The RMT has started a series of strikes on Saturdays until the end of the year on West Midlands, which includes local West Midlands services and the longer-distance London Northwestern Railway trains. West Midlands, like SWR, has pledged to keep a guard on every passenger train, but failed to assure the union.