Some main line trains were not stopping at the Olympics-serving station of Stratford today....because of the heat.
And Tube services to and from Heathrow were disrupted on the busiest day for Games' athletes arriving at the west London airport.
The main line trouble affected Greater Anglia services in and out of London's Liverpool station.
Due to heat affecting the overhead power lines Greater Anglia had to impose speed restrictions and this meant nine services were not stopping at Stratford in east London today.
Network Rail (NR), which is responsible for tracks, said that the nine were out of a total 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 £200 million 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 today are tried and tested and are designed to minimise both the risk of damage to equipment and trains as well as any disruption to passengers."
Bob Crow, general secretary of the RMT transport union, said: "While we fully agree that safety must be paramount we refuse to believe that this extraordinary move (the speed restrictions) on a major route on the eve of the Olympics is not linked to the Government's systematic cuts to rail maintenance and renewal budgets and staffing.
"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 inevitable consequences that we have seen today."
On the busiest day for athlete arrivals at Heathrow, a signal failure at Hatton Cross meant there were no Piccadilly line Tube services between Hatton Cross and Heathrow's Terminal 5 for around two hours in the middle of the day.
Earlier, services on London Overground between Euston and Kilburn High Road had to be suspended while a faulty track at South Hampstead was fixed.
There were also morning rush-hour queues on some London roads where changes had been made to road layouts ahead of the full introduction tomorrow of Olympic traffic-only Games Lanes in the capital.