Firefighters have been tackling a blaze at the Avon Valley Railway attraction at Bitton in Bristol.
A derelict train carriage stored in the sidings was set alight in the early hours of Saturday.
Avon Fire and Rescue Service said it had not ruled out arson as the cause of the fire.
A spokesman for the attraction said the 1935-built carriage was being stored in readiness for restoration, but that it had been completely destroyed.
"It was quite an old carriage, it was a former London, Midland and Scottish corridor composite coach," said development coordinator David Cole.
"It was owned by a member here and was due to be restored, but it's a write-off.
"All that's left is the chassis and metal ends, everything between is no longer there."
The attraction itself is still operating normally, he added.
