About 300 Sunday train services across Scotland were cancelled due to a shortage of drivers linked to an ongoing pay dispute.
ScotRail has apologised to customers for the disruption, which comes just one month after it was nationalised.
Last weekend driver shortages affected 110 services.
But since then Aslef has balloted its members for strike action after rejecting a 2.2% pay offer. The union said it had been left with no option and urged ScotRail and the Scottish government to get back round the negotiating table.
David Simpson, ScotRail service delivery director, encouraged passengers to check their journey before travel.
He said: "We are experiencing a driver shortage, which means some services will be cancelled.
"We are very sorry for the disruption and inconvenience this will cause and understand customer frustration when this happens.
"Unfortunately, since the drivers' union Aslef announced it would ballot for strike action, a significant number of drivers, but not all, have declined to make themselves available for overtime or rest day working."
Mr Simpson acknowledged rest day working is "entirely voluntary" but he added it means the operator does not have enough drivers to run a full timetable.
He added: "We are currently reliant on drivers working overtime or on their rest days because of delays to training new drivers caused by the pandemic.
"This is something experienced by all operators across Britain."