Zhisong Wang, a professor of physics at Texas A&M University in College Station and at Fudan University in Shanghai, China, has engineered a molecular locomotive. Powered by nothing more than light, the machine would chug along a miniature track and deliver molecular building blocks with nanometer accuracy.Before a ticket can be booked on the molecular express, though, there's the small challenge of building it. So far, the locomotive exists only in theory. Wang, who described the device in the Sept. 15 issue of Physical Review E, points to earlier work at the University of Munich in Germany on single-molecule optomechanical energy conversion as a good place to start in the development of an actual micromachine.The theoretical locomotive is a molecular chain with specific optomechanical properties. A laser would be fired at the locomotive, causing the leading edge to undock from the track. Firing a second pulse would cause the chain to stretch, reaching the next node on the track. After a brief period, the free end would reattach to the track. Another laser pulse, operating at a different wavelength, would undock the trailing end. A final pulse would shrink the molecule, and the free trailing end would reattach. In this fashion, the locomotive would have moved forward, inchworm fashion, by one node.Repeating the process would send the locomotive forward, and changing the sequence of pulses would cause it to reverse direction. Cargo could be tethered to the molecule via a bond, enabling the precise movement of freight. Because an external laser would control the locomotive, it would be fairly powerful.As for building this little train, demonstrating a few steps should not be difficult. The challenge will involve continuously and automatically running the locomotive.