Quick answer...
Coupler tensile strength.
UPRR's link to the Locotrol site noted this on their graph, but didn't put in layman's terms.
Using your example of SD70ACe's on one train, a COAL train, and using guestimate figures:
SD70ACe produces 135,000 lbs of tractive effort at 10 MPH. Couplers can withstand 400,000 lbs of draft force (pulling). A particular line's ruling grade calculates the maximum trailing tonnage of a coupler at 22,000 tons, and the SD70 is rated at 7500 tons per unit on that grade.
The example 100 car coal train weighs 13,100 tons and could "conventionally" make this run with 2 SD70's (7500 x 2 = 15,000 ton rating). Some new manager decides to add 25 more cars to this coal train, for 125 cars and 16,375 tons. The tonnage is BELOW the coupler rating of 22,000, but OVER the two units' 15000 rating. Sooo...the man adds a third SD70 to the headend consist for a 22,500 rating. BUT...3 units making 405,000 lbs of tractive effort EXCEEDS the coupler's draft strength of 400,000, which means busted knuckles and a mad Conductor. Engineers must be cautious when faced with these kinds of loads. This is where a DPU could be used. Two head end units and the one on/near the rear. The following week, another new manager decides to max out HIS coal train at 170 cars, 22,270 tons. NOT. The coupler will break because the ruling grade dictates a max trailing tonnage of 22,000 for that line. So, the man cuts five cars off, for 165 @ 21,615 tons. He slaps 3 SD70's on the head end, and off they go ... BUT once again, 405,000 worth of tractive effort = busted knuckles/mad Conductor. Time to add a DPU...
Each line's ruling grade dictates how many trailing tons can be handled. Simply putting 3,4,5 or 8 units on the HEAD end doesn't work; the coupler tensile strength as well as tractive effort must be adhered to when calculating loco consists and tonnage. The thought of 5 SD70ACe's on the head end of a coal train is, well, not very good. 125 cars, 16375 tons, just getting the train moving would probably snap the knuckle ... 5 units @ Notch 3 (85,000 lbs effort) would be 425,000 lbs of tractive effort (exceeding tensile strength of 400k) at 5 MPH. Bad news. Equally bad would be 5 SD70ACe's on the head end of a 8000 foot, 4500 ton intermodal train. Because as soon as you release the air brakes and go to Notch 1, those 5 units would pull the slack out too fast and snap the knuckle anyway, even though you're no where near max tonnage/tensile strength. The "trick" is how to deal with these issues when faced with them; and that's what being a Locomotive Engineer is all about ... you CAN run 3 SD70ACe's on the head end of 16,375 tons without breaking the knuckles.
Theoretically, properly placed DPU's would negate any coupler tensile strength issues and allow for "unlimited" tonnages and train lengths.
