TfL added that contingency arrangements were in place in case of faults on that section of the route during the strike, although repairs could take slightly longer than usual.
RMT said Cleshar staff had been offered a 1.5% pay rise, which it described as well below the Retail Prices Index measure of inflation, and said the company had not improved its offer despite talks.
The union added low basic pay was encouraging excessive overtime and contributing to fatigue among staff in safety-critical roles.
It also raised concerns about alleged underpayment of overtime and annual leave, as well as changes to sick pay and paternity pay compared with arrangements under the previous contractor.
RMT general secretary Eddie Dempsey said: “Cleshar has failed to table a new offer and has left our members with no option but to take strike action.
“We remain ready to reach a negotiated settlement, but that requires Cleshar to come forward with a serious and reasonable offer.”