I'm surprised I had not heard of this before:
This week, the U.K. has been quietly making transit history: it’s just brought the country’s only battery-powered passenger train into service. The train, fitted with lithium phosphate and hot sodium nickel salt batteries, is now undergoing a trial run shuttling passengers on a 12-mile stretch to the northeast of London. You can see a video of it below (spoiler: it looks like a regular train). If it works as it should, it will be able to make its journey without any connection to electrification.