I've been told that in most cases, it is much easier to upgrade track
than to rebuild it from scratch. Consider the fact that as long as there
are tracks there -- even if they are restricted to very low speeds --
you can bring in trains of rip-rap and ballast to upgrade the structure.

You can also bring in on-track machinery to replace ties or to change
out rails. And, of course, it is much easier to bring in long sections
of welded rail by train.

Yes, it is possible to do new construction with welded rail, by dragging
sections of welded rail off the end of a rail train with a bulldozer and
then bringing it into alignment with track construction equipment, but
it is much much easier to do the work if existing rails are still in place.

Consider the rebuilding of the Stampede Pass line (some of which I saw in
person). Most of the existing track was replaced with heavier rail. But
the new rail could be brought in on rail trains. The change-out of rail
and ties was then done with highly mechanized equipment.

-- Ernest