The carriage, number 641, was one of a set of LNER coaches dating from the 1930s which were damaged by vandals in July 2017.
They had been restored by a conservation charity and were being used on the North Yorkshire Moors Railway. The wrecking spree took place overnight while the carriages were in sidings near Pickering Station.
The eight coaches were out of action for the rest of the 2017 summer holiday season, but 641 was more severely damaged than the rest and has had to undergo extensive repairs before her return to service on the heritage line this weekend.
Well-wishers donated around £20,000 to help get the coaches, which appeared in the ITV period drama Downton Abbey, back on the rails.
Windows, kitchen equipment and furniture were all broken shortly before the NYMR's annual 1960s event.
Owners the LNER Coach Association, which is run by volunteers and maintains old carriages, sent 641 to restoration experts LS Engineering at their depot on the Ecclesbourne Valley Railway for a major overhaul.
She underwent external bodywork and lighting repairs, furniture reupholstering, and was given a new interior, flooring, tabletops and windows.
The other seven coaches returned to service around a fortnight after the incident. Two 17-year-old boys were arrested at the time.
Speaking in 2017, the railway's general manager Chris Price said:-
"We were absolutely devastated to discover that the carriages had been damaged overnight. The set was found with the windows smashed and the furniture and fixings completely ruined, obviously all the staff and volunteers are extremely upset."