What’s the ‘tractive effort’ of a locomotive?

What’s the ‘tractive effort’ of a locomotive?

The ‘tractive effort’ is a measure of how large a load the loco can pull and set in motion from a standstill — the maximum force it can exert at the drawbar or coupling. While the raw horsepower rating of the loco is important, it is not the whole story. The loco's weight also comes into play, as a heavier loco can pull a larger load without its wheels slipping. Once the wheels begin slipping, the force that can be exerted by the loco drops dramatically. (Slipping occurs more with the front wheels because the front of the loco tends to lift slightly due to the reaction torque exerted by the rails on the loco.) Modern locos tend to have electronic slip control to control the power applied to each axle separately to minimize slip and maximize the tractive effort under different conditions.

Source - IFRCA.org