The big thing is not the cost of the driver it is the capacity. As the article says: "The main benefit of driverless trains is capacity, as the computerised co

The big thing is not the cost of the driver it is the capacity. As the article says: "The main benefit of driverless trains is capacity, as the computerised control systems can allow trains to run closer together, which increases passenger capacity on the railway." However as the article says this isn't as big a gain on the Underground since it is pretty optimized already and gains would be minimal for the cost. For most metro-type rail systems driverless is the way to go though, it can offer increased capacity and lower running cost (especially off-peak).