South Western Railway (SWR) says it will run more than 60% of services during planned strikes next month.
Members of the Rail, Maritime and Transport union (RMT) are due to walk out on 8 and 9 November.
The operator, which took over the franchise from South West Trains in the summer, has published a contingency timetable showing replacement buses on some routes.
The RMT accused the firm of resorting to "desperate and risky" measures.
The union is in dispute with rail companies over driver-only operated trains which it claims would be unsafe and lead to job losses.
SWR said it would deliver about 70% of the normal number of seats on its peak services to London but warned services may be busier than normal.
RMT general secretary Mick Cash said: "RMT remains deeply concerned that 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 around the table with the RMT, working out a solution to this dispute which underpins the guarantee of a guard on their trains."
SWR managing director Andy Mellors said: "This strike is unnecessary. We have repeatedly assured the RMT that we plan to keep two safety-trained employees on our trains.
"No one will lose their job. We have been very clear that we are guaranteeing the jobs, salaries and terms and conditions of guards."
The 48-hour strike will be also be held on Southern and Greater Anglia's networks, while Merseyrail and Arriva Rail North staff will walk out for 24 hours on 8 November.