Keeping trains apart is crucial to safety

trains colliding requires knowing where they are. In olden days this was done by the handing over between driver and signalman of a token showing that a block of track was occupied. Now, automatic devices detect and report a train’s passage. But the principle is the same. Lines in a railway network are divided into blocks, and only one train at a time is allowed in a block.Rationing space this way can, though, lead to inefficiency. More precise information of each train’s whereabouts would permit trains to travel closer together without compromising safety, and therefore allow more services to be run. But what might seem the obvious approach to doing this—to employ the satellite-based global positioning system () or one of its equivalents—is not actually suitable. is unavailable in tunnels. And where several sets of tracks run in parallel (for example, at junctions) satellite-based systems can have difficulty distinguishing which track a train is on, with potentially catastrophic consequences. But Martin Lauer of the Karlsruhe Institute of Technology, in Germany, thinks he has an alternative.