Others have pointed out driverless metro is already a thing in some places (from memory Singapore MTR, Sydney Metro, Docklands light rail in London, airtrain at

Others have pointed out driverless metro is already a thing in some places (from memory Singapore MTR, Sydney Metro, Docklands light rail in London, airtrain at JFK airport (NYC), Vancouver SkyTrain). But let's say for regulatory reasons you need a driver. So the way this would work is the electric motors in both the driver operated train and the slip carriage are electrically coupled via the rails. Technically the one driver is operating two trains with a single controller, but the circuitry is set up in a way that the second the slip carriage is always responding to the controls identically to the main carriage +a few m/s until it gets close enough for mechanical coupling. So the driver has no mental overhead in managing the secondary train sets. From a transfer of momentum perspective, his train behaves as if the slip carriages are already mechanically part of his train. For the sake of example, I'll focus on my second use case. A mainline heavy rail that branches into multiple light rail at the same service frequency. In this case, the carriages split off from an 8 car set into two car sets and coast to a stop at designated branch station (like old fashion slip carriages from the late 1800s). At those stations new drivers board to take over light-rail style operation. When the drivers complete the route they step off the train at which point the automatic reverse slip carriage system rejoins them into an 8 car train (one driver stays on to become the lead carriage). The drivers that stepped off then cross the platform to take over control of the next train to split itself. A strange but doable logistic coreography that lets metro lines branch more without intolerable service frequency.