I too am curious about this. I have seen footage of an Alamosa bound train leaving Cumbres. The first dozen or so cars after the 498 were empty flats. The next 15 cars (or abouts) were tanks, (presumed to be loaded based on direction of travel) and then another dozen or so empty flats.
Wouldn't the tanks do better directly behind the engine in this situation. The profile from Cumbres to Alamosa is mostly downhill then flat, but there has to be a few spots of and and down that might create slack action.
Thanks,
Kevin