Why do railway tracks and some bridges have gaps in them?
All materials, but especially metals, will expand when they are hot, and shrink when they are cold. If the railway tracks had no gaps between sections, they would buckle in the heat, and cause a crash. Lintel bridges would also buckle if they were big enough, and not divided into sections with gaps. Modern bridges of this type usually have the sections on rollers. Telegraph lines need to sag because being taut could cause them to snap if they shrink in the cold.
