As far as I'm aware, the only hump yard in Mexico is Tlalnepantla in Mexico City. It is now run by TVM (Terminal del Valle de Mexico), a neutral terminal railroad established when the regional concessions were set up a decade or so ago. TVM appears to be doing quite well; I talked to one of the senior managers at a conference in June.
The only South American country with a well-developed rail network is Argentina, but (perhaps due to the multiple track gauges) I'm not aware of any hump yards there. The same is true in Brazil, which in addition to multiple track gauges, tends to have railroads focused on single-product movements (Vale de Rio Doce, for example). I did some work for Ferronorte a few years ago, which is building a rail line north from Sao Paulo state into the Mato Grosso, and they're handling mostly unit trains of soybeans for export.