SDL39s were custom built for the MILW - basically a four foot shorter GP39 on lighter export type C-C trucks, whereas SP's SD39s were some of the heaviest 6 axles on the railroad in their time, and SP regularly assigned them (single unit) to the Lone Pine branch which was still the original 75# rail for most of the line beyond Searles. At 125 tons, the SDL39s were more in line with the weight of a four axle road unit. Being so light, they would have to derate to 2000 hp at 10 mph to keep from being too slippery. MILW also had some six light six axle ALCos. In the US, the growth of freight car size and gross weight basically made the point of having lightweight engines moot. Most covered hoppers and other cars quickly grew from 110 tons, to 125, to 131 and now at 143. The only real common cars less than 143 gross tons being built are intermodal, auto racks and some specialty cars. The 70 ton (net capacity - 110 gross) boxcar of the 70s are quickly being retired (in part because of the 50 year limit), replaced by TBOX/FBOX designs. Ever notice the size difference between the old PFE mechanical reefers (also just about gone) and the new jumbo reefers?