When people think of trains, they often think of the distinctive clickety-clack sound caused by a train as it rolls along the rails. This sound is caused by tiny gaps which are built between sections of railroad track, as people who have inspected rail tracks closely may have noted. These gaps are deliberately placed to ensure that the tracks will not buckle in extremely hot weather when the tracks expand.
The distinctive gap in railroad tracks hasn't always been there. Initially, designers of railroads butted the tracks right up against each other for a smooth ride. However, they noticed that in the winter, tiny gaps would form between sections of track, and in the summer, tracks would deform and buckle, increasing the risk of derailment. This is because metal contracts in response to cold, and expands in response to heat.
When rail companies realized the problem, they started including gaps in railroad tracks, which allowed the metal to expand in warm weather without buckling or tearing up sections of tracks. However, the gaps must also be designed to accommodate shrinkage in cold weather, because if the gap gets too large, it can cause navigational problems for the trains which pass over it, especially in the case of extremely heavy trains. Therefore, the size of the gap must be carefully chosen, taking the specific properties of the metal used into account.
These distinctive gaps are known as expansion joints. Depending on where they are built, the gaps in railroad tracks may be staggered, so that the train only pass over one gap at a time, or they may be evenly matched. Staggered expansion joints are most common in the United States. Because of the stress on the tracks at the site of an expansion joint, railroad tracks must be carefully inspected on a regular basis to ensure that the bolts joining sections of track are not coming loose.
Because the gaps in railroad tracks can cause problems and they are difficult to maintain, some railroads prefer to use welded tracks. These tracks must be specially welded so that the tracks do not grow weak at the site of the weld, as the metal will continue to expand and contract in response to the weather. Welded tracks are used for high-speed trains, where a smooth ride is of the essence, and they are growing increasingly common, thanks to the development of superior welding techniques.