A driverless train needs to be entirely grade-separated, which generally restricts it to metros. Big cities generally have legacy metros where it would be diff

A driverless train needs to be entirely grade-separated, which generally restricts it to metros. Big cities generally have legacy metros where it would be difficult and expensive to update signaling to allow automated operation. And driverless metros generally still need a train operator to handle the doors and passenger safety, so the train/signaling upgrades are mostly worthwhile when the line is so busy that being able to run a couple more trains per hour pays off. And ATO requires a level of competence in operations and maintenance that not all organizations are capable of: https://en.wikipedia.org/wiki/June_22,_2009_Washington_Metro_train_collision https://en.wikipedia.org/wiki/June_22,_2009_Washington_Metro...