This is a Sailing Train

That's right. This train is not powered by a steam or diesel engine, but by the wind. It's called a sail bogey. This was a Nineteenth Century form of transportation in windy areas, especially those close to the sea. It often took the form of a boat that was mounted on a railway chassis.
You can see a photo and a description of another one in northeastern England here. Such sailing bogeys were, I gather, more recreational than practical.
I suspect but am not certain that the sail bogey photographed above is from the historical Ffestiniog Railway in northwestern Wales.