>>1011053cont'd
In medium sized cities the issue is different, and usually boils down to an impossibility of offering the same level of rapid transit coverage that a large city can achieve. This can be clearly seen in the failure of the german Stadtbahn concept, which ended up being impossible to fully convert into true subways, but instead left those cities with a hybrid that neither offers full subway capacity in the tunnel sections (thus not achieving the tunnel's full capacity), and complicates operation in the surface sections.
Despite maybe having similar densities, the smaller size translates to lower demand for transportation even in proportion to density. That means subway systems will be limited in their coverage, since they can only be justified in the most densely used corridors. In this situation complementary tram systems make even more sense, not just for capacity, but for offering a reasonably good service over the whole city, instead of having a few subway lines which offer excellent service, and everyone who doesn't have a good subway connection has to use buses which offer terrible service.
Examples of this scheme are Vienna, which is an excellent example of trams and subways complementing each other, also Berlin although it's a strange case because of the east-west differences, Milan also has a small-sized subway complemented by a large tram network, Amsterdam, and a few others. Notice how these usually have 1st gen tram systems. Again, there's no objective reason not to emulate the metro+tram scheme, and I can only conclude that it's been the pressure from lobbygroups that have prevented this scheme becoming more popular.
