Reductions could include on the main Edinburgh-Glasgow line, reducing trains from eight to four coaches, The Scotsman understands.
Other routes between the two cities could be affected, along with some cross-city lines in Glasgow.
Further cuts to services are also being considered after they were reduced from about 90 per cent to 80 per cent from November last year.
This is because there is likely to be significant pressure to cut costs as ScotRail and Caledonian Sleeper will together cost nearly £1 billion to run over the year to March as a result – twice as much as normal.
Staff being affected by Covid has also forced some new cuts, such as on the Stranraer line.
Alex Hynes, managing director of Scotland’s Railway, which comprises ScotRail and Network Rail Scotland, outlined the plans to MSPs today.
He told the Scottish Parliament’s connectivity committee: “We are now operating 80 per cent of services for about 10 per cent of the customers.
"We don’t think that’s very sensible.
"We are actively looking at operating shorter trains where possible and we are also looking at timetable reductions.
“We want to make sure we being a good and efficient operator and we’re saving the taxpayer’s money.
"But we also need to make sure we’re providing enough seats for the customers that we are seeing – about 30,000 every day – and allow those customers space for physical distancing.”
Mr Hynes said any changes would take account of passengers being able to stay 2m apart, even though the requirement was 1m so long as they wore face coverings.
He said: “We make calculations to say we need to give every customer travelling 2m physical distancing, and we size the rain on that basis.
"We would only shorten train services if the number of people travelling were able to physically distance at 2m.”
However, Mr Hynes also said: "But I think it’s likely that in the coming weeks we will reduce the length of trains and indeed the timetable because of these latest restrictions, which no one is expecting to be released soon.”
Mr Hynes said that where possible, early and late trains would be retained for shift workers among those doing essential jobs.
ScotRail suspended or truncated four services a day on the Glasgow-Stranraer line from last Saturday after signallers were forced to isolate because of Covid-19.
It clarified today: “The reductions were introduced at the request of Network Rail to ensure a reliable service was maintained whilst they were experiencing higher than normal levels of absence amongst their signallers.”
A spokesperson for the Scottish Government’s Transport Scotland agency, which controls the ScotRail franchise, said: “The rail industry is currently considering the impact of enhanced lockdown measures as they apply to rail travel across the country.
“Any decision to shorten the length of selected trains will only be taken following a review of passenger numbers to align capacity with demand and also mindful of over-arching Scottish Government guidelines and legislation.”