Add ability to make different bridges around the rail as placed.
Trestle under track
beam (aka over road) [as a new block type that gets autoplaces by existing system?]
truss/cantaliever above track
etc.

Try using the Rail Bed Fill feature. It will allow you to choose a block to put under the track automatically for making bridges and filling gaps.

Items can be added to the in game list by adding it to the "railBed" ore dictionary. They must have a solid top face.

If that is insufficient, comment here and re-open the ticket

I was thinking of 2 different modes. 1. for regular rail, a block added, to be used in rail bed, to make it look like http://ceephotos.karcor.com/wordpress/wp-content/uploads/20110323-amead-Railroad-Bridge-over-Seaway-Dr-Muskegon-MI.jpg

for 2nd mode , having a straight+bridge track, that adds truss, like http://multimedia.scmp.com/news/china/article/One-Belt-One-Road/img/london/04_kazan_volga.jpg
or places spans/arches and support columns under, like http://www.lewisbogatyphotography.com/img/s/v-3/p2010895143-5.jpg

I am not planning on adding those sorts of buildable structures. as part of track laying
Look at other mods like RFtools for copying a pre-built section multiple times