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

Have you ever watched a mile-long freight train rumble by and wondered how one locomotive can pull more than a hundred fully loaded cars? The locomotive weighs maybe 150 metric tons, and each car is about 100 metric tons, which means it’s hauling 10,000 tons.

I mean, if you weigh 170 pounds, this would be like pulling three SUVs totaling 12,000 pounds. Ridiculous, right? I’ll give you a hint: It’s not about weight or mass—at least not directly. It’s about friction, which is the resistance to motion between two surfaces that are in contact.

Friction gets a bad rap—we use it as a metaphor for something that hinders productivity. But without it, things would not go smoothly. You couldn’t walk; you couldn’t even tie your shoes. You’d drop your latte. Your bicycle tires would spin in place and you ...