Note how the superstructure rests on two short piers near the middle of the bridge. The haunched girders must have been carefully shaped to ensure the superstructure is balanced on these two central piers. I imagine that the bridge was constructed by cantilevering outward in both directions from this fulcrum.
Also note the key at the top of the piers, which was meant to provide transverse stability to the superstructure. It seems more likely that the tiny key would break, or that the superstructure would topple. However, the fact that the bridge has survived several earthquakes suggests that these keys are effective at preventing transverse movement.
Mexico's Bridges: Viaduct Beltran (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.