Train Weight vs. Tractive Effort?

Question:  Could you answer a question on how power is assigned to a train?  Example: I have seen mention of a locomotive set producing 240,000lbs of tractive effort, but that's only 120 tons, and I usually see mention of trains weighing 6,000 tons. So I don't understand how the math works here.

Answer:  A locomotive only needs to pull hard enough to overcome the rolling resistance of the train.  As long as the force applied exceeds the resistive force, the train will accelerate.  Modern roller bearing trucks on freight cars offer very little rolling resistance.  You can also think of the analogy of you pushing your car on a level street.  You don’t have to exert anywhere near the car’s 2000 lb weight to get it moving.  In fact, once you get it moving, you only need to match the rolling resistance to keep it moving at constant speed.  This is analogous to the starting and continuous tractive effort supplied by a locomotive. 

As an aside, coupler knuckles are designed to be the mechanical fuse in the train (it’s way easier and cheaper to replace a broken knuckle than a broken draw bar or ripped apart draft gear).  Most modern North American coupler knuckles are good for somewhere between 350,000 lbs and 650,000 lbs of tensile force.  This is why, on steep grades, you need to push a train from the rear instead of simply adding more pulling power to the front.