The vandals broke through wood in the corner of one carriage to get inside, and kicked the doors in on the others, according to the trust.
They also broke open toolboxes, a computer used in the restoration process, and left graffiti tags on both the inside and the coverings of the carriages.
Penson, 80, said since 2013 there has been about four or five cases of vandalism on the site.
He expects the latest incident will cost about £4,000 to repair.
“That’s money we can ill spare,” he said.
“It’s just a disappointment, a big sigh, and pick up the pieces when we can.”
