If a train is too heavy to make it up a grade, the train may stop at the bottom of the hill, uncouple a portion of the train that the engines can handle, and take that portion to the top of the hill (after securing the portion left). They will put the first portion in a siding or yard at the top or other side of the hill, secure it, then return to the rear portion of the train. They will then pull the rear portion to the first portion and reassemble the train. The train will provide flag protection while making these moves to prevent another train from getting between the head and rear portions. It generally did not require any additional orders to double a train, since all the moves were done under flag protection. Was a relatively common in the steam era.