The term LOCOMOTIVE MUST is composed of two words. The first word, LOCOMOTIVE, is a four-letter word meaning a self-propelled vehicle used for hauling freight or passengers on a railway. The second word, MUST, is a four-letter word meaning to be obliged or required to do something. Together, the two words form a phrase that suggests that a LOCOMOTIVE is required to move along the railway. This phrase is often used to describe the importance of locomotives in the transportation industry.