How Can a Locomotive Pull a Long Train That’s Much Heavier?

Driving wheels generate traction via static friction with rails static friction between driving wheels and rails can have a high coefficient (example given 0.74 for steel-on-steel).

Wheel axle bearings produce much lower kinetic friction when roller bearings and lubrication are used kinetic friction coefficient for unlubricated steel-on-steel is about 0.56; with roller bearings and lubrication it can be around 0.002.

Locomotive maintains traction by using static friction rather than allowing wheels to slip locomotive can throttle to maintain static friction while opposing wheels spin on the other side.

Train wheels roll on rails rather than slide rolling wheels contact the rail at one point and roll to the next point without continuous sliding.

Train mass creates a large normal force on rails a very large train mass (example given 10,000 metric tons) produces a very large normal force (example ~100 million newtons).