A question came to me today and I really wonder why I never thought of it before. What engineering factor(s) determine the point of connection of a steam engine's main rods to the drive unit? Example (based on four hours of consulting my limited literature): Consolidations, Mikados, Berkshires, Mountains and Northerns all have eight drivers, yet the first three classes seem to have the main rod connected to the third set of drivers while the second set of drivers is utilized for the last two groups. It doesn't seem to be a simple function of driver diameter or era. Can anyone provide an explanation?
Seeing I'm inquiring about rules, permit me to also ask about exceptions. I know that the Santa Fe had a class of Atlantics in which the first rather than the second set of drivers was employed. IIRC Moguls have used both the second and third set of drivers. Does anyone know if exceptions exist for the examples given in the first paragraph?
Thank you in advance for your aid.
Jerry