Gresley conjugated valve gear

The Gresley conjugated valve gear is a valve gear for steam locomotives designed by Sir Nigel Gresley, chief mechanical engineer of the LNER, assisted by Harold Holcroft. It enables a three-cylinder locomotive to operate on with only the two sets of valve gear for the outside cylinders, and derives the valve motion for the inside cylinder from them by means of levers (the "2 to 1" or "conjugating" lever and the "equal" lever).

Source: Wikipedia — Gresley conjugated valve gear (CC BY-SA 4.0)