Motive power Vs. tonnage

The tonnage ratings of 1st & 2nd generation diesel locomotive have been lost to the pre-computerized archived.  Current ratings for several territories

Territory                SD402  SD50 C40-8 ES44DC SD70AC CW44C CW60AC
WB Mountain          2100   2500  2700   2900      3450       3650       3950
EB  Mountain          2900   3450  3750   4050      4750       5050       5500

Flatland                  8550 10150 11100 11950     14100     14950     16200

All the engines have their ratings of continuous tractive effort, which is supposed to determine the maximum tonnage any diesel can haul over any given piece of dry track. For a 62:15 F7 that's 40000 lb; for a 62:15 SD40-2 it's 83000 lb. So the SD is supposed to be able to match a pair of F7s. Can it always live up to that? Us fans don't have much idea about that.

It's a different question what the different engines can haul at, say, 50 mph.

From The Railroad- What It Is And What It Does  by John H Armstrong

"Over most of the speed range, tractive force is limited by engine horsepower, and the curve is the same for both types of units.  The four-axel unit will actually haul a little more tonage because it has some 55 tons less of its own weight to pull along.

The difference shows up below about 23 mph, where adhesion required by the lighter unit to develop tractive force corresponding to its horsepower begins to reach the limit or reliable traction -- 16 to 18 percent. To keep the engine from being "slippery" its control circuitry is arranged to cutback its power at lower speeds.  The six-axel unit thereby moves out in front in haling capacity..  With 50 percent more traction motor thermal capacity, the C-C unit can "lug" that much more tonnage up a grade where low -speed horsepower is needed."

 Here's an example from my territory - an 8600 ton grain train required 5 sd40-2s or just two c40-8s - big difference! The easy calculation of horsepower vs. tonnage is gone now with all the advances in adhesion management and AC traction which have greatly increased the tonnage a locomotive can pull - at least until the computer fails then all bets are off and its back to sand,feel and listen for engine speed and wheel noise and hope you can make it!

I have found this website to be helpful in answering these type of questions:

http://www.alkrug.vcn.com/rrfacts/hp_te.htm

BB-61

we base power for trains on Tons per axle,where 1 axle = 10,000 lbs of tractive effort.  i.e. a SD40-2 has 7.1 equivelant axles, computes out to 71000 lbs tractive effort, a GP 40 has 50000 lbs, for 5 axles

C40-8 is 10.1 (101,000)

B40-8 7.8

GP38 4.5

GP39 4.5

SD70 10.4

C44-9 11.5

C44AC-12.1

C60AC 12.1

SD90/43 MAC  11.6

SD80MAC  13

SD90MAC  12

SD70ACE  12

 At CN we use horsepower per ton. 2.2 hp per ton to get trains over the hills on the old BC Rail. CN will add power or remove tonnage to ensure the Hp/T meets the requirement for the sub.