Re: counter balances and speed.

In case you did not realize, the 470's 480's and 490's all have 44" drivers. The main reason the 470's could run faster is their rod assemblies (main rods, side rods, etc) are much lighter and need much less counter balance. With a smaller drive wheel (less than about 52") it becomes difficult to fit enough counterwieght onto the wheel to properly counterbalance a heavy rod assembly. This is obvious in that 470's have only a outside counter balance on the main driver and the big mills have them on all 8 drivers - as well as on the driver center on the #3 drivers.
Another "go fast" aspect to the 470's was the trailing truck under the back of the cab, a more rigid trailer truck design that anchors the rear of the engine down and reduces swaying of the cab. a 480-90 has the trailer truck axle about even with the front wall of the cab leaving about 10 feet of engine suspended to the rear. On a curve, the front end goes one way while the back of the cab goes the other - just like riding in the back of the school bus! Also 470's carried a bit more percentage of thier wieght on the pony truck which lead them into curves better (at a reduction in pulling power). Last but not least 470's are lighter and take holes in the track with much more grace.