Part 9/12:
Instead, the locomotive pulls the first car, which then begins to move once static friction is overcome. The coupler's slack allows the second car to engage and start moving individually, followed by subsequent cars. This sequential engagement, known as slack action, means the engine never has to overcome the full static friction of the entire train simultaneously. As a result, a single locomotive can start a train of over a mile in length traveling at mere 5 miles per hour—even before the last car begins to move.