Train drivers are now learning to use new technology that allows them to recycle energy which is either sold to the National Grid, or passed to other trains.

High speed means high energy. An accelerating train can consume electricity at a rate of 2,500kW so drivers are learning to put something back. More trains are having equipment fitted so that when they slow down, the motor becomes a generator and produces electricity instead of using it.

This is relatively simple with trains that run using overhead power lines. The electricity here is AC, and the generated power can simply be sold back to the National Grid. Companies have been doing this for some time.

But it's taken a lot longer to tackle the problem of how to do it with trains that use a third rail to supply their electricity.

Many of these trains are used by commuters in the south of England. These use DC electricity at lower voltages which can't be returned to the grid.

Many Southern and Southeastern trains now generate electricity with their braking and instead of returning it to the grid, they put it back into the third rail network - for other trains in the area to pick up and use.

Elsewhere, London Midland has just installed a simulator at their base in Birmingham so drivers can learn how to drive with maximum energy efficiency.

A display shows the optimum energy use and generation, and drivers can see their performance next to it.

Essentially it involves accelerating and braking as gently as possible - but within the confines of the timetable.

I'm told the optimum eco-driving between Birmingham and Wolverhampton adds around a minute to the journey - and if trains are already late then there's less emphasis on driving in the most efficient way possible.

Train companies can save around 15% of their energy use in this way - which is like one in seven trains being powered for free.

In London Midland's case the savings were factored into their franchise bid so they say they don't suddenly have a pot of money to reduce fares with.

Modern trains have increased their energy demands with things like all-electric doors.

Campaigners say this new technology is welcome, but want it to be adopted by all train companies as quickly as possible - especially on the stopping services where there's the most to be gained.