Locomotive is a machine that moves trains on railroad tracks. It is sometimes called a railroad engine. Locomotives of the early 1830's weighed from 5 to 8 tons (4.5 to 7.3 metric tons) and could pull or push only a few light cars. A modern diesel locomotive may weigh over 210 tons (190 metric tons). Modern locomotives often use multiple-unit operation. In multiple-unit operation, two or more locomotives are linked together under the control of a single engineer. Several modern diesel locomotives working together in multiple-unit operation can pull 200 or more loaded freight cars at one time.