Trusses are structures consisting of fracture critical members namely the truss chords are fracture critical. This means that if a chord fails the whole bridge fails i.e. collapses. Trusses come in come in three forms; Deck, Through, Pony Trusses.
Deck truss members are below the riding surface or deck.
Through truss members are above the riding surface or deck, and the entrance to the truss has an overhead member.
Pony Truss has the riding surface separated by the chords and they are below and above the riding surface without a top.