The world's oldest and most famous transport system overflows with sweaty commuters on a hot summer's day.

But for all the technological advances in the world, nothing will stop the rising heat surrounding the London Underground train network. And now the streets above could be at risk too.

That's because heat emitted from trains running under London's streets for more than a century has baked the surrounding clay.

The clay that envelops tunnels is dangerously close to the limited amount of heat that it can absorb, Sharon Duffy, head of transport infrastructure at Transport for London Engineering, said in a report.

Only 2 percent of heat on London's trains is caused by commuter body heat, she said. Instead, the majority of heat in tunnels arises from movement by trains, especially when they brake and slow down.

Some of London's larger tunnels were originally built with holes across the ceiling to accommodate steam trains, pushing heat upwards. Nowadays, these tunnels accommodate air conditioned carriages. For the rest of the ageing network with smaller tunnels, however, removing heat has been possible in only a few locations.

With the temperatures in central London tunnels averaging 32 degrees C (90 degrees F) in busy rush hour periods, authorities are experimenting with potential solutions on a limited scale.

For instance, London's transport body has installed water chillers to blast cold air onto one platform beneath St Paul's Cathedral in the capital's financial district.

While the act of removing all heat from tunnels is virtually impossible, Transport for London Engineering aims to stop heat getting there in the first place.

London is not alone. New York's subway platforms can exceed 37.8 degrees C (100 degrees F) on a regular basis, according to New York radio station WNYC.

Documents obtained by Politico's Capital New York showed that concerns about high temperatures on the city's transport network were raised by a panel spearheaded by New York Governor Andrew Cuomo in 2014.

"At higher temperatures, expansion joints on bridges and highways are stressed, and the instance of rail track stresses and track buckling increases," the draft report noted. "Underground, subway platforms and stations could become dangerously hot for riders."