Agree comment section is interesting, well worth a browse if you have time. Lots of comparisons between loop and undergound, light rail, rail and bus networks. It looks like on most metrics, loop is better (an order of magnitude in the case of cost).
There are a few things that were not mentioned.
1. Loop gives point to point, there is no need to change.
Take the example of London, about 600 square miles so that means there are 180,000 pairs of mile square routes. The London Underground network has about 272 stations with each station connected to about 25 other stations along the line, so about 7,000 pairs. Add in rail, light rail and tram and there are perhaps 15,000 pairs of mile squares that are connected directly, for the rest to go from one mile square to another you must change, perhaps more than once.
Compare that to a hyperthetical Loop network for London. Each square mile would perhaps have 10 loop stations connected by 20,000 miles of tunnels. Point to point would be possible for all of London. Such a system would have a capability for about 20 million passenger journeys per day and a peak of maybe up to 4 million passenger journeys per hour.
2. Using cars that can travel on city streets as well as in tunnels means that door-to-door travel is possible. Not only does this cut down journey times, but makes it easier for many types of disabled users and parent with small children to use public transport. Some people (mainly elderly) are put off using public transport because of percieved risks of being attacked, particularly at night, not only on public transport but also on the walk to the station.
3. Compared to subways or even ground level streets it is much easier, quicker and cheaper to alter the network as usage changes, adding in stations and tunnels.
