TRAINS GET EVEN BETTER:

Hydrogen trains have already replaced more polluting diesel engines on a line in Germany, and some train companies think the vehicles could be running in Britain as early as 2022. Introducing them would still require substantial investment and wouldn't be without challenges. But they could be an important step towards reducing the carbon footprint of railways.

Only around a third of the UK rail network has been electrified, with little extra track converted in the last few years. Without continuing to electrify the network, the government is faced with the dilemma of how to eliminate diesel trains that produce carbon dioxide and other harmful pollutants.

The current strategy is to purchase bimodal trains that can switch to using diesel when they reach parts of the track without electricity. But this is fudging the issue of dealing with climate change and air pollution and still leaves the UK well behind most other European networks.

The hydrogen gas would need to be compressed into tanks that would usually be stored on the train's roof. But adding a regenerative braking system to charge an additional small battery would reduce the amount of hydrogen needed to power the train.If electrifying the rest of the network is deemed too expensive, one potential alternative is to generate electricity on board the train. One way to do this is to use fuel cells that combine hydrogen gas with oxygen from the air to produce electricity and water. Hydrogen can carry more energy than the same weight of batteries, meaning fuel cell systems could be lighter. They also take less time to refuel than batteries take to recharge and don't have the same high environmental costs from manufacturing.

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