Ian Craig with a sheet of UV-resistant film which is going to fitted over the windows of the Edinburgh trams. Picture: Jon Savage

Its windows have been covered with ultraviolet-resistant film to keep down the temperature inside.

The experiment was launched after the 27-strong fleet was ordered without the option of air conditioning for passengers.

Travellers complained about uncomfortable conditions on board last summer, when temperatures reached 25C.

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That led to tram doors being opened at every stop to try to combat the problem.

The clear film is being assessed for its effectiveness.

However, it has already been a hit with tram staff, who said it made a big difference.

Edinburgh Trams is also looking at other options, including installing more “hopper” windows that can be opened by passengers. Further modifications are being considered, such as using the air conditioning system in the driver’s cab to help cool the rest of the tram.

Fan speeds could also be increased to improve the flow of fresh air through the passenger areas.

Officials said the fleet could also be fitted with air conditioning, but the city council would have to fund the £60-80,000 cost per tram, or a total of £1.7-2.1 million. However, they said the equipment could be installed without cutting back services.

Interim tram director and general manager Ian Craig said: “It is only an issue on exceptionally warm days, but the temperature inside the tram can be 8C higher than outside. We have installed heat sensors in the tram with the UV film, and the early signs are that it is making a difference - and the staff like it.”

One passenger, John Whighorn, said last year: “When the sun shines and the trams are full, they are not a nice experience. The only windows are quite small and the ventilation system didn’t seem to work at all. It was horrible and sticky.”

Lawyer Daniel Donaldson, who campaigned for an inquiry into the shortcomings of the tram scheme, said temperature control should have been addressed at the start.

He said: “Any move to improve the quality of the passenger experience would be welcome, but it goes back to the lack of planning from the outset.

“My biggest concern is that it was not conceived as a problem.

“A lot of public buildings have film applied to their glazing to absorb radiation.”

The city council said potential upgrades would be considered further when the results of the current trial were known.

A spokeswoman said: “Edinburgh Trams is currently testing ways of improving ventilation in trams efficiently and effectively.