Well, I don't know the specifics and I'm sure some of the other more knowledgeable guys will chime in soon, but here is my take on things. The railroads are a little more advanced than you guys give them credit for. When I worked for CSXT we were all issued certain books and inside the Timetable for any particular division it included a Tonnage Rating Chart. This chart included each subdivision and a list of locomotives and ratings. For example (this is made up as I don't have a timetable handy)
Popes Creek Subdivision
Bowie-Brandywine
SD40, SD40-2, SD45, SD45-2, C30-7, C36-7 2200 tons
GP40, GP40-2, B30-7, B36-7 1800 tons
Brandywine-Pope
SD40, SD40-2, SD45, SD45-2, C30-7, C36-7 2000 tons
GP40, GP40-2, B30-7, B36-7 1550 tons
Pope-Brandywine
SD40, SD40-2, SD45, SD45-2, C30-7, C36-7 700 tons
GP40, GP40-2, B30-7, B36-7 450 tons
The above is not actual numbers and may be way off. All locomotives approved for use on that subdivision were listed. The was also a list of locomotives prohibited from moving over the division for various reasons such as clearances, axle loading, etc.
Now, you have your tonnage ratings for the locomotives, the paperwork has the tonnage for your train, so you know what power to assign. So, a 10,000 ton coal train going from Bowie-Pope needs 5 SD40-2 or a equivalent. Notice going into Pope has much higher ratings than coming back out of Pope, but the outbound train would be empty so the same power would be satisfactory.
Railroads also have a maximum number of powered axles. On CSX it was 24 on the Baltimore Division. It was later changed to allow 27 axles on the Popes Creek sub because we needed 3 SD80MACs for the pull up Duley's Hill. SD80MACs and other AC units are counted as more axles than they actually are because of the tremendious amount of tractive effort. 80MACs are 9 axles. 3 units = 27 axles online.
When you see 7, 8, 9, or more units in a train chances are only the first 3 or 4 are working. They do things a little different out west so I don't know. With a 24 powered axle limit that is only 4 SD40-2s, I know years ago the Popes Creek trains ran with 5 or 6 of them so things have changed some in recent years.
And as mentioned before the couplers rating can be exceeded if great attention isn't being paid to the tonnage of the train. We used to run 170 car coal trains from Brunswick to Baltimore with 3 up front and 2 shoving, 20,000 tons. Sometimes putting all the power up front is not the best option.
And one more thing the "Power Desk" is the group that decides what power to assign and also handles power balancing and shop transit moves. Usually they follow the numbers but sometimes they don't and when the engineer tells them he'll never make the ruling grade with the power he has and they tell him to just do what he's told. Usually this situation ends up with the train stalling, expensive delays and having to be shoved. Most engineers know very well the capabilites of the equipment on their division.
Dave