Competition on public transport is limited by the fact that you often need a cohesive network to get from A to B via C & D. If multiple companies cooperate on

Competition on public transport is limited by the fact that you often need a cohesive network to get from A to B via C & D. If multiple companies cooperate on crossover ticketing and align their schedules, yes, it can work. Even more likely to work is a model where a central authority opens tenders for specific services, and transport companies can compete on that. Then again, I don't think it's viable to build subway tracks and tunnels to compete. Even if you have space under the earth, there may not be space for more stations, and good station locations are important too.