South Western Railway (SWR) services on the Hounslow loop will be cut by 40% during planned strike action on November 8 and 9.
The RMT union is staging a 48-hour strike as its dispute with SWR over the future of guards and the extension of driver-only operated trains continues.
RMT leaders say the strikes, announced on October 24, are a response to "attacks on the role of the safety-critical guard and the extension of driver-only operation (DOO)".
The union maintains SWR should uphold an agreement made with predecessor South West Trains to guarantee two members of staff on all services.
SWR managing director Andy Mellors has branded the strikes "unnecessary", denied any extension of DOO was planned and insisted SWR would keep two safety-trained employees on its services.
"This strike is unnecessary," he said. "We have repeatedly assured the RMT we plan to keep two safety-trained employees on our trains.
"This is because we know a second member of staff provides assurance to our passengers and ensures we have capable colleagues on board to deliver assistance or offer advice during journeys.
"No one will lose their job. We have been very clear we are guaranteeing the jobs, salaries and terms and conditions of guards.
"In fact, as we expand our fleet, we will be hiring more drivers and guards to help deliver more and better services for passengers."
RMT general secretary Mick Cash said: "The RMT remains deeply concerned SWR are prepared to cut corners on safety in order to run trains with staff who are ill-equipped to carry out the full range of functions of the guard.
"Instead of resorting to these desperate and risky measures, SWR should be round the table with the RMT working out a solution to this dispute which underpins the guarantee of a guard on their trains."
SWR said it expects to run 60% of its services during the strike and has announced a contingency timetable detailing the changes.
Rail replacement bus services will be in operation on some routes, but none in west London.
Southern Rail will also be affected, but is expecting to run a "normal service on most routes".
The operator said in a statement: "Southern will do everything they can to minimise disruption to your journeys and it is expected that a normal service will operate on most routes.
"Journey planners are expected to be updated with the revised timetable approximately 48 hours in advance."
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