The UK’s first 100mph battery-diesel hybrid train is entering passenger service to cut carbon emissions and boost air quality - and it's on the Chiltern Railways line.

It was developed by adding a powerful battery to a 20-year-old diesel train to reduce fuel consumption and CO2 emissions by 25 per cent, according to owner Porterbrook.

The firm added that the two-carriage train, named HybridFLEX, also provides a 75 per cent decrease in noise and a 70 per cent decrease in nitrogen oxide.

The government has pledged to remove all diesel-only trains from UK railways by 2040.

Chiltern Railways will introduce the train on its 40-mile route between London Marylebone and Aylesbury, Buckinghamshire, from Thursday, which means it will run via Harrow-on-the-Hill.

There are plans to use it between the capital and Oxford in the coming months.

The operator previously had a fully diesel fleet.

Chiltern Railways managing director Richard Allan said: “Chiltern Railways is determined to operate a railway that is as sustainable and environmentally friendly as possible.

“We have worked hard with our partners to fit a powerful battery power pack underneath a 20-year-old diesel train to make the train cleaner, quieter and quicker.

“We are really proud that this concept train is now carrying customers and look forward to assessing its performance in daily service.”

Porterbrook chief executive Mary Grant said the entry into service of HybridFLEX is “a significant first step” in demonstrating how the trains can slash emissions and boost air quality across the rail network.

Warren East, chief executive of Rolls-Royce, which produced the battery, described the project as “groundbreaking”.

He said: “In this critical decade of climate action, today’s entry into service of the HybridFLEX train demonstrates what we can achieve through technological innovation and agile collaboration.

“This smart piece of engineering enables the acceleration of the UK Government ambition to remove all diesel-only trains from the network by 2040, making rail journeys quieter, cleaner and faster.”