The reason you have not seen one is that none ever existed. There were some double track low trestles. Three come to mind - Southern Rwy. over Lake Pontchartrain, SP over the Great Salt Lake (Lucin Cutoff), and LIRR over Rockaway Bay. The unrealistic track spacing of Ntrack demands that single track bridges be used, so you'd need three single track trestles. Think about it, a double track bridge with 13 ft. centers requires 22 ft. long ties. A triple track bridge with two 20 ft. centers would require 49 ft. ties! The cost would be horrendous.
So, concentrate on three separate bridges, it'll look much more realistic.