Services on a key transport route to the Olympic Park were suspended on Tuesday after a fire alert.
The problem meant no trains were running on a section of the London Underground's Central line, the main transport link from central London to Stratford tube station, which serves the Olympic Park.
To add to Olympic travellers' difficulties, rail services in and out of Liverpool Street main line station were affected by a signalling problem.
The Central line was suspended between Liverpool Street station and Woodford/Newbury in both directions.
"The suspension is due to a train being taken out of service at Leyton after the driver reported smelling smoke," a Transport for London (TfL) spokesperson said.
The Central line was delayed by a rush hour fire alarm
"This has been investigated and all passengers left the train and station without reported problems."
TfL advised Olympic spectators to use other Tube and main line services to reach Stratford, including the Underground's Jubilee line, the London Overground and the Docklands Light Railway.
The main line difficulties at Liverpool Street were caused by an overhead wire problem between Elsenham and Stansted Mountfitchet in Essex.
This was affecting travellers on services run by the Greater Anglia and CrossCountry train companies.
The problems around Liverpool Street come on another busy day for public transport as the first weekday evening football match takes place at Wembley Stadium.
The Central line serves as the main route to the Olympic Stadium, in Stratford
This is only one of a number of Olympic events taking place in the capital, with TfL advising that London is expected to be exceptionally busy throughout the day.
TfL said people were heeding advice to avoid London if possible.
It added that while Tube journeys were up 4% yesterday, road journeys in London were down by 20%.
London's Transport Commissioner, Peter Hendy, said: "Public transport in general, and the Jubilee line in particular, will be exceptionally busy today as spectators make their way to events at key Olympic venues across the capital.
"We are very grateful to customers for following our advice to avoid London Bridge station and for major businesses who are enabling their staff to work flexibly during the Games.
"Our advice to all users of the transport network continues to be to plan ahead, avoid hotspot areas, and leave plenty of time for your journey."