Have you ever wondered why some railroad bridges across rivers are so very high, while automobile bridges are quite low? The trains look a little scary, rushing along way up in the air. But there’s a good reason why they do it, and those tall trestles are so wonderfully planned and built that they are very safe.

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

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!

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.

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.

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.

That’s where CTC comes in. Trains cannot bump into each other when CTC is at work. It is a wonderful system of electric wires that run along the tracks, all the way to an office building in a railroad town. The wires end in a long board that’s dotted with lights and small levers. Now when train wheels travel over the rails, the wires carry electric messages to that long board. Lights flash on and tell the man who watches the board exactly where the train is. If he wants it to go onto a siding, he pushes a lever. Electric switches miles away guide the train’s wheels off the main track. At the same time, signal lights tell the engineer to stop.

What’s more, CTC has extra safety machinery, just in case the man at the board makes a mistake. If he pushes levers that might make two trains bump into each other, stop signals go on all along the line. All trains come to a halt until the mistake is corrected.

In the old days, trains that ran through western ranch country were often late. The crew who had orders to pull onto a siding knew they might have to wait a long time. So they could just take a walk to the nearest house, wake the rancher and settle down for a visit. If their host was in a good humor, he’d build a fire and cook them a meal. Then, when they heard the whistle of the approaching train, they’d start back in plenty of time to signal as it passed their siding. Railroaders have fun talking about those early times, but they’d really rather have the safety of Centralized Traffic Control.

CTC helps to keep passenger trains moving safely into big cities, too. The man at the board—he’s called the dispatcher—decides which track each train should use. He pushes the levers. Electric switches move. Signals flash to the engineer, and lights on the board show every train moving along.