Lachlan Valley Railway secretary/treasurer John Healy said the rocks had damaged windows at the front and on a side door of one of the group's 'Tin Hare' rail motors.

The train is nearly 100 years old.

It is one of three rail motors being stored at the rail yards in Orange.

Mr Healy said the damage to a piece of toughened glass in front of the driver's seat and the side window caused about $1500 damage.

He said Orange police had told him two young people had been cautioned over the incident which occurred on February 19.

"They were caught and issued a warning by police," he said.

Mr Healy said the train had been due to run a trip from Dubbo to Coonamble from the Country Championships event next Sunday but this had now been cancelled.

He said they had been unable to source replacement glass and have a glazier install them in time for the trip.

Mr Healy said it was the first time the train had been damaged by vandals while it had been stored at Orange.

"We've had worse at other places," he said.

A statement on the LVR's Facebook page said people who had booked for Sunday's trip would be refunded.

"Replacements parts will not arrive and be installed in time to have the old girls ready for service for the run to Coonamble," it said.

"We hope to reschedule in October for the 2020 Gold Cup."

The LVR has a long-term plan to build an operational depot at Orange East Fork which would include a large enclosed shed for storing trains and the group's equipment.

Mr Healy said Orange City Council had offered to assist the group with its plans including helping develop a presentation of the project to seek grant funding from government agencies.

He said he would be meeting council staff at the site this week to discuss the project.

Mr Healy said the group also needed more trackwork to be completed at East Fork to allow its trains to access the site.

Volunteer crews have been clearing the site.