The move comes after the operator was forced to cancel hundreds of trains since Saturday due to staff catching the virus or coming into contact with others who had tested positive for coronavirus.
Caledonian Sleeper has halted all of its London to Fort William services between Sunday and Friday this week, while CalMac had to suspend Largs-Cumbrae sailings on Sunday.
ScotRail is looking at potential service reductions across the Central Belt after introducing a temporarily-reduced timetable for last Friday because of Covid absence, in which more than 100 trains were halted.
Staff shortages forced around unplanned 150 cancellations on Saturday and Sunday, some 60 yesterday, and some 40 today.
A total of 50 of its 1,060 drivers – or nearly one in 20 – were off for Covid-related reasons at the end of last week, which has since reduced to 37.
They are among a total of 113 staff off for such reasons.
Friday’s temporary timetable involved suspending all services on the Glasgow-Ardrossan Harbour and Kilmarnock/Ayr-Stranraer routes.
Trains between Glasgow and both East Kilbride and Paisley Canal were both reduced to hourly, while there were around 15 cancellations on the Glasgow-Kilmarnock line.
A ScotRail spokesperson said: “As part of contingency planning, we are looking at various options to amend the timetable in order to provide customers with some level of certainty over what services will run should the number of Covid-related absences rise significantly.”
Magnus Conn, Serco’s operations director for Caledonian Sleeper, said: “Due to recent increased levels of absence of safety-critical staff, we have taken the proactive decision to make planned cancellations to our northbound Fort William services until Friday.
“It is our priority to provide certainty to guests, but the safety of our guests and staff is always of utmost importance.
"We are very sorry for the inconvenience caused to all of our guests affected by this disruption and have contacted these guests to offer alternative arrangements.”
CalMac said it had been forced to withdraw the MV Loch Tarbert ferry from its Largs-Cumbrae route on Sunday morning after several crew tested positive for Covid and a number of their colleagues were identified as close contacts.