Shane sent in this brief article to share:

A runaround track allows a train to move in front or at the back of another train or locomotive. It is usually used at the terminal of a branch line to prevent the train from reversing direction.

A runaround track easily switches between carriages. Switches are installed at both ends, allowing a locomotive to push another car into a point facing spur by going around it in parallel. This is useful when trains return from a trip and drop off cargo. Using engines at both ends of a train eliminates the need of ‘running around’.