Overheated passengers on the Hammersmith & City Line will soon be able to take advantage of London Underground’s first air-conditioned train.

Mayor Boris Johnson, who boarded the new air-conditioned train at an Oxford test track, said passengers will be “terrifically impressed”.

He said: “For thousands of clammy Tube passengers some relief is finally in sight. We have now begun testing the first of 191 super cool and spacious new trains.”

All the trains using air-conditioning will operate on the subsurface lines with the first running on the Metropolitan line, followed by the Circle, District and Hammersmith & City.

However, the cooler trains will not be in service until next summer. And for commuters using the deep level lines, such as the Victoria, Central and Northern, it will be years before they get relief from the sweltering conditions.

These lines were built long before air conditioning was developed and there is no space for such bulky equipment in the narrow tunnels.

The Mayor said: “Cooling the deeper lines remains a considerable challenge. A crack team of Transport for London engineers is focused on that and is concentrating on the Victoria line in particular.”

The Tube's 3.5 million daily users face yet another long, hot and very sweaty summer with in-train temperatures expected to reach as high as 47C which can cause some passengers to pass out.

Measures to keep the ageing network cool have been hit by funding cuts. Instead Tube bosses will resurrect their old campaign of advising passengers to carry bottled water with them, not board a train if they feel unwell and to get off at the next stop if they start to feel ill.

• What do you think? Let us know in the comments section below.