Group Manager for Greater Manchester Fire and Rescue Service, Andy Heywood said: "At the moment we are applying water to the outside of the building to stop the fire spreading.
"By allowing the fire to burn inside the building it stays hot and gives off smoke which is relatively clean.
"Unfortunately because the building and fire is right next to the railway line we have asked for the trains to be stopped and overhead line equipment isolated.
"It could be several hours before we can be confident of what will happen to the building and how the fire will develop."
More top news
An alleged victim of Barry Bennell stayed at his house a court has heard.
Remaining cold and wintry. Best of any sunshine Saturday. Possible hill snow Sunday before turning milder
Drones are seized at a rate of more than one a week at HMP Liverpool and conditions are described by inspectors as the worst they've seen.