Hundreds of thousands of rail passengers faced travel misery on Monday at the start of a series of strikes in the long-running dispute over guards on trains.

Members of the Rail, Maritime and Transport (RMT) union on South Western Railway (SWR) walked out on the first of 27 days of industrial action lasting until New Year's Day - the longest stretch of action against a major rail operator in living memory.

Passengers have been warned that only around half of services will run, including those to and from London Waterloo, the country's busiest railway station.

Services will be cancelled, replaced by buses or finish earlier than normal, and trains that do run are expected to be busier than normal.

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Monday's disruption was worsened by a track circuit failure which hit services between Reading and Ascot, meaning fewer trains were able to run on all lines. The problem was expected to hit services until midday.

Talks between the two sides collapsed last week, with the union and company blaming each other for the failure to reach a deal.

SWR managing director Andy Mellors said the company was running more than half its normal services due to the strike action.

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He said there had been further disruption on Monday morning after police had to board a train in Woking due to an incident unrelated to the industrial action.

Mr Mellors told the PA news agency: "We are running over half of our services today, so clearly there is potential for some customers to have trains that are busier than usual.

"But we are here at Waterloo station and passengers are moving freely."

Steve Hedley, RMT's assistant general secretary, said about 900 union members were striking across the SWR network on Monday.

About 30 union members formed a picket line outside Waterloo station in London next to a large banner that read: "Keep the guard on South Western Rail. This strike is all about safety."

Digital signs at the station also warned passengers that travel would be "difficult" during the festive period due to the strikes and that SWR would be running an amended timetable.

Mr Hedley said he hoped the union and SWR could reach an agreement over the issue of train guards.

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He said: "Three weeks ago we thought we had this boxed off. We had a deal that was acceptable to the company and acceptable to us.

"The guard would be in charge of doors and dispatching the train. And then, for no reason we can discern, that was pulled from the table.

"Our people don't want to lose 27 days of pay, but they have really come to the end of their tether."

Rail passenger Steve Range, 63, from Essex, said his train did not appear to be affected by the strike.

He said: "I thought I would get here and it would mayhem, but it's not too disastrous.

"Luckily, I am not always using SWR, but if I was the strike would be a pain."

RMT general secretary Mick Cash said: "RMT is angry and frustrated that a set of proposals that would have guaranteed the safety-critical role of the guard at the point of despatch, and which would have cost the company absolutely nothing, have been kicked back in our faces.

"Our action goes ahead from Monday in defence of passenger safety and accessibility and the blame for that lies wholly with SWR and their wrecking strategy. The union remains available for talks and we have a deal to solve this dispute which is cost-free for SWR worked up and ready to go."

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An SWR spokesman said: "We have done everything we can and more to meet the RMT's outdated demands with our promise of a guard on every train, and a safety-critical role for that guard.

"Throughout negotiations we have tried repeatedly to find ways meet the RMT's aspirations. However, every time we find a way forward on one point, the union has moved the goalposts by changing its position.

"They said at the outset this was about keeping the guard on the train - that is exactly what we have offered.

"They said they wanted a safety-critical role for that guard - that is what we have offered.

"Unfortunately it is clear to us that the RMT is unclear on what this dispute is about and intent on striking no matter what.

"We know the impact these RMT strikes will have on our passengers and we want to be clear that we have done everything we can to try and avert the strikes and meet RMT's antiquated and changing demands."

Sean McKee, director of policy and public affairs for London Chamber of Commerce, said: "With peak services under increased pressure due to so many cancellations, and long queues expected at busy stations, it's sadly inevitable that this cynically long strike will have an impact on commuting workers and businesses during peak commuting times.

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"With trains due to finish earlier, it will also impact shift work and the night-time economy - key to the festive economy."

Mr Cash said the start of the strikes was being "solidly supported", with picket lines mounted outside stations across the SWR network.

He added: "This strike is solely about protecting safety and accessibility on SWR trains. The stakes could not be higher.

"Instead of spending a fortune mobilising an army of under-trained and potentially dangerous contingency guards, the company should be back round the table with the union concentrating on reaching the negotiated settlement that is easily within grasp, which they committed to verbally in earlier talks and which would cost SWR nothing."