Imagine a steam locomotive built for maximum traction for rural rail tracks of the old days. You could not add weight to add traction like you would normally do, but you could use the whole weight of the locomotive and spread it evenly on the whole area so the track would not collapse. This is the basic idea behind the shay type of steam locomotive.

To achieve the even weight distribution some quite unusual solutions needed to be found. It would be uneconomical and technically very difficult to spread the pressure of steam evenly through the 3 bogies of the shay using the traditional design. What you need is a cardan axle, which is the same kind of solution that is found on many modern trains and most of the backwheel drive modern cars. Except that on the shay the cardan axle is on the sides probably for easier handling and maintenance. To drive the cardan axles, the steam cylinders were located right in front of the driver. And finally you need the gearing to operate heavy loads of logs on steep grades, so shays trade power for speed.