Imagine you have a sports car. If you are going slow, you can floor the gas pedal and the engine will provide so much torque to the wheels that they just start spinning and you burn/peel out. The traction of the wheels is not high enough to prevent this. The available traction can be expressed as a force (I'll get back to this down below). However, if you don't have a high-performance sports car, but a used 4-cylinder with fewer liters than a quart of milk, the engine might not be able to output enough torque to the wheels to overcome traction. Just like traction, we can express the final output of the engine as a force instead of a torque (again, more about this below).
Now, for both types of cars, if you're already going at a high speed, then flooring the gas pedal may not do anything at all. In fact, if you're going fast enough, you can actually start to slow down if you go up a hill even if the pedal is floored. The engine simply cannot provide enough force to overcome wind resistance, rolling resistance, frictional losses,
and gravity. Crucially, the power output of an engine is related to the amount of torque (and thus force) that is available to the vehicle. Obviously an engine with a higher maximum power output would be able to drive the vehicle at a higher top speed, but, more importantly, an engine with a higher maximum power will be better able to handle hills and slopes or changes that increase the required towing force (we don't usually run vehicles at their maximum speed, so that extra power can be used to overcome gravity or those changes).
Next, imagine that you hook up a trailer or two to your car. All of the various resistive and frictional losses in the trailers can be changed into a single value: the force required to pull them. And, if you've done any physics before, you'll remember that accelerating an object requires a force (f=ma) and that the required force increases as acceleration increases. So accelerating an object at twice the acceleration requires twice the force.
Remember what I said about traction and engine power being expressed as a force? This is where it comes in. We can combine both traction and engine output as a single value: tractive force. Tractive force is simply the lower of the two. The amount of tractive force determines how large of a load you can hook up and accelerate. Higher tractive force means a larger force available to tow with and a larger force means you can tow a larger load or accelerate the same load at a larger acceleration.
Here the wikipedia article on
tractive force, and the paragraph about railed vehicles:Remember that tractive force takes into account both the amount of traction
and the torque/force the engine is able to output. This means that tractive force generally decreases as speed increases, as the graph in the article shows.