London Midland is advising its passengers not to travel between Milton Keynes and London Euston today.
A major fire at a commercial property in Harrow means mainline train services cannot currently run to the capital.
A spokesman for London Midland said "While the emergency services are dealing with the fire it simply isn't possible to run trains through the area. The fire closed the mainline around 0030"
London Midland is also reminding passengers delayed by 30 minutes or more to claim compensation. Details can be found on the company's website at www.londonmidland.com/delayrepay .