The Heathrow Express was suspended on the busiest day in the London airport's history - after a fire close to the train track.
Tens of thousands of tourists, official Olympic sponsors and VIPs were advised to use alternative transport, which included taking taxis into central London or travelling on the Piccadilly Line. Services have now been resumed, according to a BAA spokesman.
Some corporate sponsors are bringing tens of thousands of guests to the Games.
But most of the international leaders and VIPs arriving at the Games today will be using Luton or Stanstead airports - as Heathrow does not allow charter or private jets.
Some corporate sponsors are bring tens of thousands of guests to the Games. Heathrow expects a record 125,000 passengers, including 2,000 Olympic officials.
The BAA spokesman said: "The Heathrow Express and Heathrow Connect services experienced minor delays this morning following a fire at a platform of West Ealing station. Services are now back up and running."
Earlier on Thursday morning, cable thieves left travellers and commuters stranded for up to 90 minutes as they headed for London on First Capital Connect trains. The train company said they were "devastated" by the delays.
London's roads have been "quieter than normal", according the Transport for London commissioner Peter Hendy.
“Traffic levels yesterday were around 13.5 per cent lower than they would normally be at this time of year, due to motorists heeding our advice about avoiding driving in central London, near venues or on and around the ORN.
"Londoners have yet again proved how sensible they are, and I’d like to thank them for their help in reducing traffic in London – which is now truly an Olympic city.
TFL recommended drivers continue to avoid the A40 and A3 because of the Wimbledon Olympic lanes and the A12 and A13 close to the Olympic park. Those roads, and the M4, have experienced long queues since the opening of the Olympic Road Network.
The London Organising Committee for the Olympic games have predicted around 1,300 vehicles an hour will use the lanes at peak times.
Roads between Westminster and Surrey will be closed for road cycling events over the weekend.