Well, for starters, you need a good story why all the bridges are not the same. Looks like to me that the wood trestles are being replaced as they age, and now there is only one left that needs replacing.
Since the bridges are on a curve, I am thinking that deck girder bridges would be used. If the same RR uses all three bridges, they would eventually be the same standard type used by the RR. If different RR's crossed different bridges, they could have been different, but because they need to be curved, the choices are limited.
The rear one could be a stone arch type, but it would kind of hide the scenery behind it. The middle one would be a steel deck girder. But then if the RR was double tracked when the stone arch was built, they would have built a double track type.
So I guess the rear one could be a stone viaduct or arch, the middle a steel deck girder type, and the wood trestle in the front.
But how believable is a stone arch in the desert?
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
I have almost completed the Trestle that will go under the closest opening. This will be over a deep ravine with several waterfalls. My problem is what kind of bridges will look good over or under the second two tracks. The layout is HO and the period is the last 50's. The spans are over 100' on a curve. Thanks in advance. The links above will give you a better idea about what I am trying to do.