7109 - A loco with two engines

Sentinel 7109 - Basic layout (courtesy "Sentinel" Patent Locomotives 1931)

7109 is a steam locomotive with two engines, each of 100HP, taken virtually unaltered from the Sentinel Super or Standard steam road waggon design. 

Twin tranverse engines

Mounted vertically, they appear remarkably similar to tall car engines except that they are double acting one-stroke as opposed to single acting four or two stroke.

Sentinel 100HP Engine (from Sentinel Instructions to drivers ref)

Unlike conventional steam engines with slide or piston valves, Sentinel engines are more like early internal combustion types which used poppet side-valves activated by external push-rods operated by camshafts. They are twin-cam engines, one camshaft for steam inlet and one for exhaust.

Reverser Lever (from Sentinel Instructions to drivers ref)

There are five settings for direction and cut-off: two forward cut-offs and two reverse. A central setting (Drain) holds all the valves open to enable coasting and warming through of the cylinders before movement.

Sentinel sliding Camshaft with multiple cams per pushrod
(courtesy of Gervaise)

The valve timing settings are achieved by axially sliding the camshafts. There are five cams per push-rod and the right one is engaged with its push-rod depending on the desired setting.