Tube passengers are some of the only rail users not suffering major delays from the soaring UK temperatures.
Passengers may be hot but most trains are running at speed
Only one heat-related speed restriction has so far been imposed on one of the underground's 12 lines during the heatwave.
London Underground ordered trains on the Metropolitan line to slow down for three hours on Monday to avoid buckling tracks.
Speed limits, normally 50 mph, were cut to 40 on the overground section as temperatures peaked at about 1400 BST.
Trains were travelling at their usual speed by 1700 BST as the heat subsided.
Thirty commuters on the southbound Metropolitan line faced more problems when a train was stranded in a tunnel for more than an hour due to a signal failure.
An ambulance was called to nearby Baker Street station however nobody required assistance.
A spokesman for London Underground, which maintains 253 miles (408 km) of track said: "Speed restrictions are unusual and only happen in extremely hot weather.
"We are less affected because a large proportion of our main traffic is underground and not affected by the extreme heat from sunlight."
But he admitted trains do get uncomfortable during hot weather, especially as the only existing means of cooling passengers is to keep air circulating.
"Our staff are trained in first aid and we remind people if they do feel unwell don't get on the train, go and speak to members of staff," he said.
"It's incredibly difficult, we have never introduced air conditioning on the underground because that would mean displacing heat elsewhere, into the tunnels and stations.
"We put an ambulance in standby if anyone gets stuck in a tunnel and staff are on hand with water at stations.
"We do continue to work to find a solution."