The Napier Deltic engines are triangulary shaped power plants with each bay having 3 cylinders, 6 pistons and 3 cranks. Each cylinder has two pistons that approach top-dead-center at the same time. Confusing? Perhaps. But you’ll get a better idea of the concept in this video demonstration:
Wikipedia has more on this engine with a neat little animated GIF right here.
A few Napier Deltic trains are still in service.