Freedom of information requests by the Scottish Liberal Democrats found the oldest carriages in use as part of the fleet were manufactured between 1976 and 1984.
The trains with the longest continuous use are Class 318s, used on the Glasgow suburban line, which were manufactured in 1986 and entered service the same year.
Meanwhile, Class 156s were manufactured in 1989 and have served on the West Highland and Glasgow suburban lines ever since.
Liberal Democrat transport spokeswoman Jill Reilly said: “Old carriages and trains have a place on the service, but it does raise questions about how well capital investment in Scotland’s trains has been handled over the years and whether reliance on old trains has been allowed to impede on operational efficiency.”
David Simpson, ScotRail’s service delivery director, said it is “continually working to deliver a cleaner, greener railway for Scotland”.
He said: “Although rail is already a low-carbon mode of transport, we are committed to reducing our environmental impact ever further. The massive projects that have been delivered so far – the electrification of the Central Belt, new electric trains - mean more seats and faster journeys for our customers, as well as a rail service that is better for our environment. This plan will deliver even more for our passengers in the decades to come.”
A Transport Scotland spokesman said: “ScotRail’s current fleet is that inherited from the private operator, Abellio in April 2022. All of the ScotRail Trains Ltd. fleet is currently owned by third party rolling stock leasing companies. Public ownership of ScotRail provides Transport Scotland with the opportunity for the previous approach to rolling stock procurement to be reviewed.
“The Scottish Government is committed to a rolling programme of decarbonisation and with that we are looking at how we can introduce new fleets such as electric and alternative powered trains. This will enable us to replace the existing diesel fleet with more environmentally friendly trains that attract even more people out of their cars. Any new fleet represents a significant investment therefore we must ensure they provide value for public funds.”