A case of arson is under investigation after 24 out of order Shosholoza Meyl train carriages were torched on the rails next to the Bloemfontein train station on Saturday.
The coaches had been earmarked for refurbishment, the department of transport said on Sunday.
Transport Minister Fikile Mbalula called the incident "senseless", saying it had caused an estimated R90-million in damages.
The carriages themselves are valued at about R4-million each.
"Security personnel noticed a group of young men at the yard and removed them from the property. The youth later returned and set fire to two coaches," said the department.
Mbalula further called the incident a "pure act of criminality" and said it would be dealt with "as swiftly and harshly as possible".
Train services had not been disrupted, said the department, but the fire had affected the availability of rolling stock and damaged critical cabling at the depot.
Anyone with information on the arson should contact Parkweg police at 051 507 6000 or Crime Stop at 086 001 0111. Alternatively, send information anonymously via MySAPS app.
Arrests are yet to be made.