Sorry for the incoherent title, but googling has confused me even more 
First, have a look at this very neat time-lapse videoof the replacment of bridges near the Reading (UK) rail station this past New Year’s.
OK, see those red ‘lift’ platforms (2 smaller ones for the smaller bridges, and 1 massive one for the large bridge - the small ones make their entrance about 20seconds in) What is the correct term for them? Mobile Jacks? Mobile Lift Platforms? I also saw the term ‘transportation platforms’. Perhaps Something else?
I’ve seen images of a few other bridge projects in the UK using platforms like these - they are mobile, self-contained, and apparently controlled by one man walking alongside :eek:
I don’t recall seeing these types of units in the US (then again I don’t follow every infrastructure project), but googling around you see a lot of usage of cranes to move bridges into/out of position, or non-powered wheel jack units which are towed by a tractor, or even barge (like the new Willis Avenue Bridge in da Bronx! 
- these self-contained road capable lifting platforms seem like a really efficent method of bridge replacement.