>Even if the car's were relatively crammed close together in the tunnel, I can't imagine it would be anywhere near the density you could fit on subway trains.

>Even if the car's were relatively crammed close together in the tunnel, I can't imagine it would be anywhere near the density you could fit on subway trains. Indeed. The problem always fundamentally reduces to geometry, and the only way to solve it is to increase density. People always say, "we can just build more tunnels", but you'll just end up with congestion at interchange and entry/exit points, just like on freeways today. And the more tunnels you build, the more it takes for the operating company to recoup the costs, meaning higher fares to use the tunnels. Best case scenario, this company reduces the cost of developing rail transit. Likely scenario, we just get the same awful freeway situation but below. Worst-case scenario is the unfortunate situation you described.