The problem is always density. For example, the new 345 class train that will be used on the Crossrail Elizabeth Line in London is as long as 82 Smart fortwos p

The problem is always density. For example, the new 345 class train that will be used on the Crossrail Elizabeth Line in London is as long as 82 Smart fortwos placed back to back, but carries up to 1500 people. Cars are extremely convenient and comfortable, but an extremely space inefficient way of moving large numbers of people. Even with autonomous vehicles, you still have hard limits on how many vehicles per hour you can physically move through any point.