So, I've often seen it suggested that Santa Fe, much like Western Pacific preferred to use 4 axle locomotives for the bulk of it's fleet In particular for time critical traffic like Intermodal. Hense the purchase of the GP60Ms. Of course, the fact that they skipped the 40 series and stuck with 30s, 35s, 38s and 39s gives me pause with this. So my question is, did ATSF in fact have a motive power policy like this? Obviously they bough SD40-2s and 45s/45-2s so they had 6 axle power as well. Were these for specific locations and traffic types? Obviously, this is a question about policy from the 1960s through the 80s. What was the motive power plan over that span?
Seems as though they liked four axles much earlier. Such as various F units, and older Geep road switchers. Possibily this was just a continuation? I wonder if it was because earlier use proved to be so handy, due to ability for uses systemwide? One day out on a main line, the next on a lighter railed branch? Boxcab E50
The usual answer for the 2 vs 3 axle trucks is traction. In similar sized units, the 4 axle units put more weight on the axles for traction.....if your engine weighs 200,000 lbs (an arbitrary # for the example) and you have 4 axles, each axle has 50,000 lbs resting on it.....a 6 axle unit would have 33,333 lbs on each axle. More weight generally means more traction. The flip side is that the rail can only support a certain amount of weight...so at some point you need to add an axle just to support the weight.....that was the early reason for going to 6 axles....the center axle on each truck was just there to support weight and had no traction motor.Going back to the example, if you up the weight of the 6 axle engine to 300,000 lbs, then each axle would have the same 50,000 lbs (and traction) as the 200,000 4 axle engine. Hope this makes sense. There are other factors too....6 axle locos have 50% more axles on the rail, so there's an increase in wear on the rail, and they also have 2 extra axles that have to be maintained, but other than the axles, the rest of the maintenance is virtually identical.
Dont' know if you have read this, but it might give you some insight on the GP40's. Santa Fe's Lone GP40 Mike
Have heard one of the reasons Santa Fe went to 6 axle Dash 8s in it's last years was that that there was too much wheel slip w/ the 4 axle GP60Ms.
It seems Santa Fe favored having a lot of its fleet at a similar HP/axle - all those GP30s, GP35s, GP39-2s, B23-7s, etc. and the 3600-hp 6-axles.
The Santa Fe owned a couple of GP40X which were built in 1977 and are basically a testbed for the GP50.
The problem with that logic is that 6 axle units were generally considerably heavier than their 4 axle equivalent so the axle load would be roughly the same. An SD40 would generally weigh in the neighborhood of 360,000 lbs while a GP40 would weigh in at somewhere around 250-260,000 lbs. What this meant is that a 6 axle unit would have a higher tractive effort than its 4 axle equivalent. IIRC, EMD did build some lightweight SDL-39's for the MILW which weighed no more than a 4 axle unit and that those units were intended for branch line use on very light rail. Andre
