In London, the world's first narrow gauge steam engine, Princess, was unveiled after being restored by craftsmen on the Ffestiniog Railway, where she was used 150 years ago.

BBC: Wales

Henceforth someone trying to build an engine could pluck a standard 8-gauge, 32-threads-per-inch machine screw off the shelf.

FORBES: Architect of Life

But the engine is unable to travel by rail line itself as it can only run on a two foot wide narrow gauge.

BBC: The Princess in 1871