Tractive effort is a CALCULATED rating "at the wheel rims" of a locomotive. (Unless you have access to a chassis dynamometer that can support the weight and absorb the energy of all of the locomotive's traction motors, you can see why it is difficult and costly to actually measure tractive effort. In steam days, the PRR had a chassis dyno at Altoona, and the DOT in Colorado has one.)
Drawbar pull is the force in pounds that the locomotive can exert at its rear coupler/drawbar. This is always measured, including at different speeds, using a dynamometer car coupled to the locomotive being tested. The front coupler of the dyno car has a oil filled (usually) calibrated drawhead, and records this pulling force on a paper trace and into a computer program inside the car. This calibrated coupler assembly is coupled to the rear coupler of the locomotive that is being tested.
So...
The difference between tractive effort and drawbar pull is the energy used to move the locomotive down the track. At higher speeds, the resistance of the wind becomes a factor, in addition to flange forces, etc.
Then it gets more complicated and much more interesting......
