It does seem strange that the Western Pacific, which regularly operated articulated steam locomotives with 2 sets of drivers & fairly long overall lengths, would shun 6 axle diesels. One theory is that the road's early purchase of EMD FT's, along with EMD's trade-in policy, which required trading in EMD for EMD, basically tied the road to 4 axle diesels. I've also read of concerns regarding increased maintenance costs( 6 traction motors vs. 4 ) and higher purchase prices. As I recall, WP was close to signing for SD -45's when GE cut them a good deal on U-30-B's, allowing EMD trade-ins to boot. Curvature & track profiles wasn't an issue; witness the regular use of 6 axle BN power on pool trains in the 1970's.