In uk.railway Graeme Wall wrote:
Easy enough, prefabricate the thing alongside, close the road overnight and
swing the thing into position when ready. Not exactly rocket science.

A bridge launch can be pretty quick. Some of the GCR engineering folk
visited the CTRL works in London a few years back on Xmas day to watch
one of their bridges being launched over existing rail lines. They
said that it was all over in a couple of hours - a metre or so every
few minutes.

Jim'll