The classic 1940s Superman cartoons from Fleischer Studios laid the cornerstones of Supes' pop culture superstardom. "Look! Up in the sky!" A great example of Superman battling the laws of physics is an episode titled "Billion Dollar Limited," in which he must stop a runaway train filled with the largest gold shipment ever transported. The train careens down a slope, and Superman stops it by grabbing the last car and pulling it back up the hill. It's a beautiful illustration of an inclined plane.
When you push on an object or surface (including standing on the ground), a force called the normal force exerts an opposite and equal amount of force. This is actually the force created by the microscopic compression of atoms, and it's what makes solid objects solid. The important thing is that the normal force always acts perpendicular to the surface.
With an inclined plane, like the slope the train is rolling down, the train is pushing down on the slope (due to gravity) and the normal force pushes back an equal amount. Those forces are balanced. However, gravity is pulling the train straight down, not perpendicular to the surface, so some component of the gravitational force is acting parallel to the slope, pulling the train downhill. The exact amount of that force can be calculated if we measure the angle of the slope and know the weight of the train. This calculation is expressed through the equation F = mg*sin Ɵ.
There are two things counteracting that downward force -- friction and Superman. Figuring out exactly how much force he needs to pull the train up the hill is complicated and beyond our scope here (there are different kinds of friction involved, we don't know how much the train cars weigh, and so on). One thing's for certain: The narrator isn't kidding when he says Superman is, "more powerful than a locomotive."