Some trains were unable to stop at Stratford station yesterday, which serves the Olympics – because it was too hot.
And Tube services to and from Heathrow were disrupted on the busiest day for Games' athletes arriving at the west London airport.
The "too-hot-to-stop" problem affected Greater Anglia main-line services in and out of London's Liverpool Street station. Due to heat affecting the overhead power lines, Greater Anglia had to impose speed restrictions and this meant nine services did not stop at Stratford in east London.
Network Rail (NR), which is responsible for tracks, said that the nine were out of 500 trains that stop at Stratford in a day, with Greater Anglia running a total of 1,900 trains a day.
An NR spokesman went on: "We are currently in the middle of a £200m project to completely replace all the overhead lines on the Great Eastern main line between London and Chelmsford. While the core section from Liverpool Street to Stratford has been completed, the sections of railway which have speed restrictions in place today have overhead lines dating from the 1950s and 1960s. The wire in these sections can expand and sag in high temperatures and, as a result, it is necessary to reduce speeds in order to avoid damaging the equipment and trains.
"The contingency arrangements put in place are designed to minimise any disruption to passengers."
Bob Crow, general secretary of the RMT transport union, said: "Hotter countries than Britain run trains without a problem and we have a right to know why key services are running on infrastructure that is half a century old with the consequences we have seen today."
On the busiest day for athlete arrivals at Heathrow, a signal failure at Hatton Cross, in west London, meant there were no Piccadilly line Tube services between the station and Heathrow's Terminal 5 for around two hours in the middle of the day.
Services to and from Stratford were also affected during by a signal failure at Custom House on the Docklands Light Railway in east London.
Evening services using three main London stations – London Bridge, Cannon Street and Charing Cross – were also delayed by a signalling problem.In addition, there were morning rush-hour queues on some London roads where changes had been made to layouts ahead of the full introduction tomorrow of Olympic traffic-only Games lanes.
Drivers in central London face long delays from today as 30 miles of Games lanes reserved for Olympic VIPs are activated across the capital.