Network Rail Scotland said the fire between Connel Ferry and Oban had broken out close to a site where gas cylinders are stored.
The Scottish Fire and Rescue Service (SFRS) said crews were working to extinguish the blaze in Oban’s Mill Park Road.
ScotRail said trains, which run through Dummarton, Cardross, Craigendoran and Helensburgh Upper, were starting and terminating at Taynuilt. The alarm was raised at about 08:20.
SFRS said: “Operations control mobilised two fire engines to the scene and firefighters are working to extinguish the fire. Crews will remain in attendance until the area is made safe.”
ScotRail said services between Oban and Glasgow were experiencing delays and alterations, and that valid train tickets would be accepted on selected bus services.