We investigate a new approach which has been recently introduced to construct microscopic engines whose main characteristic is the possibility to determine dynamically the direction of motion. The approach is based on the transformation of the supplied energy into directed motion through a dynamical competition between the intrinsic lengths of the moving object and the substrate. The engines are able to move translationally or rotationally and can perform useful functions such as pulling of a cargo. We discuss possible realizations and introduce some ingredients, such as turns and switches, important for creating a microscopic `railway system.'

PACS numbers: 66.90.+r, 45.40.Ln, 87.16.Nn