This would just be a guess on my part... I'm going to say that a trestle has more vertical supports from underneath , there for there are many shorter spans that are able to support the weight.
A Bridge would be supported less from "underneath" and more from the end of the spans.. and the spans would be much longer..and made of heavier material to support the weight... larger dimentional wood, steal , trusses..
A Bridge would be supported less from "underneath" and more from the end of the spans.. and the spans would be much longer..and made of heavier material to support the weight... larger dimentional wood, steal , trusses..