In all of the cities that I visited this past week, there were at least 3 metro lines each, yet only two of them had distributive ring tram systems that supplement those systems. Budapest and Vienna were very interesting in how their systems worked a bit differently from the typical hub and spoke system found in most cities. They might be interesting case studies to look at when you compare them to cities in the United States thinking about ring systems such as Boston, Chicago, and Atlanta.
Budapest and the Caterpillars
In Budapest, the ring routes do some heavy crosstown lifting, specifically, the 4 and 6 routes which run on one minute headways and carry 10,000 riders per hour per direction during rush hours (I think there might be more). It seemed at times that the Combino Supras had many more people riding them than the subways with multi-car consists.
You'll notice in the map below that the subway system operates in the traditional spoke system but if you look closer at the Pest side of the city (That's East since Buda and Pest were initially different cities) there are yellow tram rings that connect the subway stations. The 1,4,6 and 47,49 routes make crosstown connections easy. We stayed on the 4,6 lines and took them to connect with the Metro on many occasions and were always completely packed into the cars, especially in the evenings and even at 9pm at night when students and young folks were out at night.
The benefit of these rings is that with their one minute headways combined with the subway's 3 minute headways, you can get anywhere in the city faster than in a car.
The benefits of these ring routes are many. They are fast ways to get between metro stations as well as distribute people to the places in between. You'll also notice that they connect to the suburban railways as well (in green). There are also two routes that go north and south on the river connecting the top and bottom of the ring. This is where I saw the most tourists.
The trams were also designed not for commutes but urban transportation. The interiors were chair sparse and able to fit many more people for short trips.
Next i'll talk about Vienna's Ringstrasse and the above and underground tram transfers in the Strassenbahn.