Just to illustrate BOB2's comment about the cab-forwards having been set up for reverse running in regular service, I've posted the photo below (again from MR Cyclopedia Vol. 1). The eccentric lags the crank (when rods are at BDC) with respect to its relation to the the cylinders, and the rocking link has been positioned opposite the common Walschaerts arrangement so that the engine will run in "reverse" (i.e. "forward" with respect to the engineer's position because the whole locomotive is turned around) when the engineer operates the reversing lever in the "normal" manner.
In other words, the engineer doesn't have to compensate in his mind for the fact that the engine is running backward when he manipulates the controls. At least, that's how I understand the purpose of the valve gear arrangement shown here: Forward motion of the engine is achieved by "dropping" the block and radius rod in the rocking link, just as the engineer would do on a regular SP cab-behind engine. This has been an interesting discussion about something I hadn't paid much attention to before.
I often refer to The Steam Locomotive in America: Its Development in the Twentieth Century by Alfred W. Bruce, for helpful explanations, but Bruce does discuss this question explicitly.
