The water table surely has something to do with it, but they could put much of it above ground like Bangkok does (erm, Bangkok should be listed as doing ok, even if they aren't doing as well as Singapore).

China built A LOT in the last 15 years. Beijing before 2008 had line 1, 2, a couple of suburban lines (13 and another one out east), and that was it. I don't think any other country has ever built infrastructure so quickly, so it isn't really fair to compare them to China.