CSX rules require a HP to tonnage ratio of 2/1 when possible, but not to exceed 2/1 by more than minimally needed. There is also the powered axles limit rule. CSX prohibits more than 24 powered/ dynamic brake axles on any train (exception, express trains, Q090/091,are allowed 27 axles to permit 3 CW44AC's on line) DC locos have a value simular to the number of axles ie: SD40's, SD50's, SD60's all carry an axle value of 6. C40-8's, CW40-8's, ES40dc's carry 7 axle value. AC locos like CW44AC or AH's carry a value of 9, and the monster CW60AC's are 11, and older GP38's/40's have an axle value of 4 each.
So engineers must calculate between the tonnage/Horse power ratio and powered axles ratios to have the maximum available power without exceeding parameters. Divide consist HP by the trains tonnage to get ratio.
Here's a quick example: I have a container train, about 15 cars, 5000 ft and 2,200 tons. My 2/1 HP maximum is 4,400HP. I have 2 ES40DC's for power. One engine gives me 4000hp which is below the 2/1 ratio, so I can have 2 on line at 8000 hp, even tho, I'm over the 2/1 ratio, 8000 is as close as I can get to be above 2/1.
Now, same train, but now power is 1 ES44AH in lead and 1 ES40DC trailing. Since the leader is 4400 hp it equals the 2/1 HP/tonnage rating and I can only have the lead ES44AC on line for power.
A month ago, I was assigned a rather large container train, over 7300 tons. It had 3 ES40DC's. With all 3 on line, their 12,000 hp was still below the 2/1 HP per ton. If that train had been powered by 3 ES44AC engines, I would have been restricted to using only 2. While the HP/ton ratio was still below 2/1, the AC's powered axle rating (9 each) would have exceeded the 24 axles rule
It's not just about workin the throttle, brake, and blowin the horn.
On a side note here. CSX has also been using regular trains to ferry power around, instead of light power moves, to conserve fuel and crew availability
Harleys and EMDs, some of my favorite rides