John Mills, Nottingham Fire and Rescue Service manager, said the fire is "more about complexity rather than size" because it burned continuously through voids, making it incredibly hard for firefighters to find.
Flames were seen and smoke filled the foyer of the building.
As 50 firefighters tackle a huge blaze at Nottingham railway station this morning, this is what we know..
CrossCountry Trains tweeted: "Due to a fire at Nottingham station, our trains can not run between Derby & Nottingham".
East Midlands Trains said in a statement: "There is a fire at Nottingham station".
There are now 60 firefighters at the scene.
"You don't think much of it when there's a little bit of smoke and police already there, but then we were evacuated", she said.
"No trains will be stopping at the station until further notice".
The incident has caused delays across the local transport network.
East Midlands tweeted: 'Please do not travel to Nottingham station this morning as trains will be unable to run and we are unable to provide road transport to/from the station'. This means we have no service from the Forest to Clifton South or Toton Lane.
Emergency services are dealing with a large fire which has broken out at Nottingham train station.
Furthermore, there have been road closures around the station with Notts Police tweeting: 'Station St, Carrington St & Queen's Rd are closed use to a fire at #Nottingham Train Station.
"This means that services on the Nottingham/London, Matlock/Nottingham/Newark, Nottingham/Mansfield/Worksop, Leicester/Lincoln/Grimbsy, Nottingham/Grantham/Skegness and Liverpool/Nottingham/Norwich routes are disrupted".
He said passengers should check the East Midlands Trains website for the latest travel information.