I have researched the track dual gauge track from Alamosa to Antonito a great deal and as far as I can tell there were no alignment changes over the bridges on that line. The dual gauge just ran straight across. One thing to note was that this line was originally built narrow gauge and was dual gauged in 1901 so I'm sure bridges were updated at that time as well so it is possible that special provisions were designed into the bridges for the slightly off centered narrow gauged traffic but I don't know for sure and have no evidence to support that theory.
Jared