Driver-less metros have become common in Europe and parts of Asia. The economics of automated trains tend to reduce the scale so tied to "mass" transit (the largest operating expense is the driver's salary, which is only affordable if very large numbers of passengers are paying fares), so that small-scale installations are feasible. Thus cities normally thought of as too small to build a metro (e.g. Rennes, Lausanne, Brescia, etc.) are now doing so.
