Why are railway sleepers called so

The gaps in the railway tracks are meant to allow the tracks to expand during Hot summers. the rising temperature causes the iron tracks to expand (Linear expansion due to Heat). If there be no space the Iron tracks may bent, as the expansion due to heat causes enormous force. this may even cause the tracks to break. It is only to avoid all of these effects, due to the linear expansion, that a small gap is maintained between joining tracks. Depends on the climate, the strength of the sleepers and the attachment between sleepers and rails. In (northern) Europe, it's quite common to use concrete sleepers and continuous rails. Concrete sleepers are stronger than wooden sleepers, and can withstand the forces of thermal expansion and still keep the rails in place. Weaker sleepers and continuous rails, thermal expansion can force the rails out of alignment with disastrous results.