Llangollen Railway Station, already suffering from having to close because of the Covid lockdown in Denbighshire, says thousands of pounds of damage has been caused.
The vandals broke windows on two carriages to get inside and then went on a rampage, pulling up seats and tables, as well as side panels.
The heritage carriages, which date back to the 1940s, were parked just outside Llangollen's station at the time.
Police have appealed for witnesses to the vandalism, which was discovered on October 12.
General manger Liz McGuinness said: "We don't know the extent of the damage yet but they've easily caused thousands of pounds worth of damage.
"The culprits would have had to have trespassed along our lines to do what they've done."
The railway reopened after the UK-wide lockdown was ended, attracting families and rail enthusiasts after putting Covid-secure measures in place.
But Welsh restrictions on travel imposed on Denbighshire meant it had to close down again.
It is now hoped the railway will reopen as soon as possible, in time for its popular Santa special trains.