Well, really you have a mix there. It should be truss vs plate and deck vs through. Plate and truss construction are used for deck and through types. Plate types are used for shorter spans (in modern times up to about 150 ft, but maybe only half that in older days). Deck type is used anywhere there is enough clearance under the bridge. Through is only used when there isn't, due it costing more to build. (In a deck type the trains can sit pretty much on the main supports, on a through the trains must (obviously) be between them, so a wider (strong) platform is needed between the main supports.)
Good question. Now, I understand a plate girder bridge... ...and I understand a deck girder bridge... ...and I understand a truss bridge.... ....but what is a "through" bridge??
Your pic marked "plate girder bridge" is a through plate girder bridge, the train runs between the supporting girders. Your pic marked "deck girder bridge" is also a plate girder bridge. Your pic marked "truss bridge" is a through truss, the train runs through the structure rather than across it. You can also find "deck trusses" where the structure is below track level. Like Mike said, the "through" designs tend to get used where there is a need for clearance underneath, where there is no need for clearance then the deck designs tend to be favoured as they use less metal (compare your deck-plate girder and through-plate girder images) Does that help?
Ahhhh....I see now. Good explanation. Thanks. I should have known that being in the construction industry. Then again, I build buildings and not bridges.
