"They are cowards basically. Nearly all work done is through voluntary labour and people do this purely out of the goodness of their heart.
"It's difficult enough to get voluntary labour and when our work is destroyed by mindless vandals it puts people who volunteer their time off from coming back," he said.
Vandals targeted the carriage that was beside a building because they could not be seen while two other carriages were left in the open, he said.
Only two to three days of work was left on the carriage when it was tagged. Trust members reported the matter to police before cleaning the tagging.
Refurbishment of tables and windows, recladding and painting cost the trust $10,000 and Mr Bradshaw said at least half that amount would be needed to get the carriage ready for service, hopefully by Easter.
The trust owns six carriages but four are currently running.
Mr Bradshaw is a world authority on vintage rail. In Wales he oversaw the rebuilding of 12 steam locomotives, six diesel engines and 100 carriages of the Ffestiniog and Welsh Highland railways. He has also worked on railway restoration projects in Argentina and Vietnam.
In 2011 he received the Federation of Rail Organisations of New Zealand's top honour, the PS Heighton Award for Excellence, for his contribution to the nation's rail heritage.
Last year April, three youths punched and kicked him after he caught them burgling the railway shed. The bashing came just three weeks after the theft of a model train used as a donation box from the railway.
