A vertically-adjustable gantry assembly installation adapted for removal or placement of a train bridge-span of the type which spans and is supported by two piers, comprises a gantry assembly positioned on load-bearing first ground-support locations, the gantry assembly comprising a gantry and a ground-engaging vertical support and lift system, the vertical support and lift system adapted for supporting a combined weight of the gantry and a bridge span in at least one operational vertical position above respective bridge span support-surfaces of the piers including a position corresponding to a disembarking plane in which the leg portions are extended from a stowed position to an extent at least sufficient for the gantry assembly to self-liftoff the pre-installation conveyance system onto the first ground-support locations to effect the gantry assembly installation.