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’d fall over—luckily, since you’d have no brakes. In fact, all the nuts and bolts holding your bike together would fall off.

So, yes, to answer our question about freight trains, we need to understand how frictional forces work. All aboard the physics train!

What Is Static Friction?

Let’s start with something simple. Place a book on a table and give it a little nudge on the side. Just a light push—not enough to get it moving. Newton's second law says the net force on an object equals the product of its mass and acceleration (F net = ma). Since the book isn’t accelerating (a = 0), the net force must be zero, meaning all the forces are balanced. Here’s a diagram:

Let’s look first in the vertical direction: We have a downward pull from gravity, and the strength of that force depends on the mass of the book (m) and the gravitational field (g) of the planet you’re on (F g = mg). But the book isn’t accelerating downward, so there must be an equal force from the table pushing up. We call this a “normal force.” Result: The net vertical force is zero.

I know, the idea of an inert table pushing up on a book doesn’t seem very normal. Maybe it’ll help if you realize that gravity doesn’t pull you to Earth’s surface, as people often think—it pulls you to the center of the Earth. The normal force is what keeps you from plunging through the floor. (By the way, “normal” means perpendicular—it’s always perpendicular to the surface.)