Trains can’t climb hills nearly as well as automobiles can. The slopes that trains go up must be very gentle ones. Even a little bit of up-and-down grade slows a train a great deal. So the men who build railroads try to make the tracks run along as nearly level as possible. Next time you see a high bridge across a river, look at the rest of the country around. You’ll see that the river cuts deep down between two hills. The bridge is built on tall stilts that make a level path for the train from one hilltop to the other. When trains have to go up or down a very long hill, the builders have a problem. They must slope the [Illustration] tracks very gradually. In mountains this means that the tracks zig-zag back and forth, with long, wide curves between the zigs and the zags. If you look back at the picture on page 19, you will see how one railroad solved the problem. The rails are laid so that they spiral upward, making a loop. When a very long train travels along the loop, it’s like a huge snake coiled around over its own tail! [Illustration] Unless it’s absolutely necessary, the builders try not to make curves. Trains run faster along rails that are straight as well as flat. Every bend means that the engineer has to slow down a little. And so there are two reasons why railroads often have tunnels right through mountains. Instead of climbing far up and then coming down in long, slow curves, the train can run quickly straight through. Tunnels are hard to dig. They often have to be blasted out of solid rock. So the builders don’t make them any bigger than they have to. Of course, there’s not room for a man to stand up on top of a freight car as it goes through a tunnel. To protect brakemen who might forget, there is a device called a tell-tale close to the mouth of a tunnel. It is simply a fringe of cords hanging down from a tall bar across the track. The cords touch the careless brakeman and warn him to get down right away before he’s scraped off and hurt. [Illustration] If you started in the morning, it would take you till night just to name the inventions that have made railroading more safe than it was a hundred years ago. Some of them are simple things like a tell-tale. Others, such as air brakes, are complicated. The most wonderful invention of all took hundreds of scientists a long time to work out. It’s called Centralized Traffic Control, or CTC. [Illustration] To see what CTC does, you’ll first have to imagine a stretch of railroad way out in the country, thirty miles from any station. There’s just one main track, with sidings where trains running in opposite directions can pass each other. Each engineer has his train orders, so he knows whether he’s supposed to go onto the siding or continue straight through. But unexpected things can always happen. If a train is late, it may not get to the siding on time. Then there will be danger of a collision.