Efforts to squeeze more out of Sydney's rail fleet as a part of an overhaul of timetables has resulted in "smaller windows" of time available to maintain trains and tracks, putting more pressure on staff to carry out night-time work.

While extra weekly services will help the network cope with surging demand, a Sydney Trains document obtained by the Herald lists, among the challenges for carrying out train repairs, "reduced fleet maintenance windows" and an "increased demand on maintenance as [a result of the] fleet doing more kilometres".

Decades-old S-set trains are also being pressed into service more often in order for Sydney Trains to put on the 1500 extra weekly services rolled out as part of the timetable changes. It leaves passengers to face uncomfortable rides this summer because the S-sets are not airconditioned.

The leaked document, written before the timetable overhaul on November 26, also warned of "shorter maintenance windows" for the repair of rail tracks. Trains are running more regularly during off-peak periods of the day, and later at night, thereby reducing the "midnight to dawn windows" when work can be carried out.