The tractive effort of a motorized vehicle is the force exerted on the driving wheels of a locomotive or automobile. The greater the load or weight of the vehicle, the more tractive effort is needed to haul the load. A train engine needs a tractive force of 2 to 5 pounds per ton in order to move forward on a straight track, according to Republic Locomotive.
Several factors influence tractive effort. Tonnage, track curvature, percent grade and acceleration rate all combine to factor into power calculations for trains. Curved tracks need more tractive effort because axles on train cars are rigid. Uphill grades need even more force to haul a load. Greater loads require more acceleration to get a train up to the proper speed.
A traction motor is vital for trains to overcome friction and resistance in order to accelerate. Smaller locomotives for yard use provide sufficient tractive effort over straight tracks and short distances. Larger engines are needed for long hauls because they provide more horsepower.
Trains in rail yards rarely need more than 600 horsepower to move slowly. Short railroad lines need locomotives that require 2,000 horsepower. Long hauls across the country need multiple locomotives that exceed 4,000 horsepower due to high tractive efforts and higher speeds.