Fast angle wheels were introduced when wheel and axle design was changed to a rigid assembly. Previously, each wheel would rotate on the axle independently.
When a wheel/axle assembly goes around a curve the outer wheel travels a longer distance than the inner wheel. When each wheel moved independently this was no problem.
But with flat tread rigid assemblies, one wheel would drag slightly around the curve since each is traveling a different distance.
The fast angle design allows the outer wheel to ride up on the higher part of the tread effectively increasing it's diameter to compensate for the extra distance it travels. Thus, there is less drag overall on the wheel set.
Prototype wheel axle assemblies work the same way due to the curved fillet between the tread and flange. Since prototype curves are so wide, the the angle does not have to be severe as on the model.
Jim
