For around 50 years, most UK trains have run off electrified overhead wires and tracks (where they exist), or emissions-heavy diesel engines (where they don't). This week, though, a new way of making trains go has hit the network for the first time: the UK's first battery-powered trains.
The train will be trialled between Harwich International and Manningtree stations in Essex in January and February. It's not quite, as Network Rail, the government agency responsible for it, claims, "emissions free": the batteries must still be recharged using mains electricity, which across the UK is still generated from largely unsustainable sources.
There's another problem. The charge only lasts for one hour, and charging them up again takes two. So while the trains can run independently of the overhead electrified wires which now cover much of the UK rail network, they must use electrified parts of the network for two hours in every three.
All the same, they are much quieter and less polluting than the current stock, which would make journeys more pleasant for passengers. If they become widespread, the reduced pollution could even bump property prices near rail lines.
Here's a picture of the train in question.
This isn't actually a new train: it's just a standard model which has been retrofitted to run off 80,000 rechargeable standard-size batteries.
That's worth saying again: they don't use a special, giant battery, they just use lots of standard ones. Here's James Ambrose, senior engineer for Network Rail, holding one of the appropriate size. We think it's an AA:
Both images comes from this video from National Rail, which, complete with plinky plonky music, gives more details on the train and its benefits:
So if you were thinking of asking for an electric toy train set next Christmas, it might be worth setting your sights slightly higher.