Possibly over a thousand km. Or two!
VIA expansions
New VIA rail corridor is being discussed as possible target of Trudeau's government funding.
(
$4 billion dollars). This involves
several hundred kilometers of new track.
LRT projects
Several funded different LRT projects in Ontario -- Ottawa Confederation Phase 1(funded)/2(mostly funded), Waterloo ION, Hurontario, Hamilton, Finch West, Crosstown. Half of these already under construction. Future funds allocated for Sheppard East, Scarborough, and unfunded projects London LRT, Jane Street LRT, East Bayfront LRT, etc. Given the twin-track nature, this is easily, combined,
half a thousand kilometers worth. So if only a few projects come to fruititon, that's still a couple hundred kilometers.
(Note: I'm not sure how much of a proportion has been ordered already -- but I think Ottawa Phase 1, Crosstown, and ION has ordered most of their rail already. But that still leaves a whole batch of new LRT projects waiting in the wings).
GO expansions
There are major GO expansions, due to GO RER electrification ($13.5 billion). Both Barrie and Stoufville are being twinned or tripled in various locations.
Several hundred kilometers of new track will be added in parallel to existing track.
So math it out,
totalled, potentially a couple thousand of kilometers of new rail steel needed in Ontario! In worst case scenario (two-thirds cancellation), that's still several hundreds of kilometers. This doesn't include potential high speed trains which are finally now being a bit more seriously looked at, though probably won't begin construction late 2020s or early 2030s at the current glacial pace.
IMHO, this is sufficiently large scale enough to at least
look at starting-up new rail steel manufacturing.
Now a good business case analysis is needed to see whether it's economically feasible to do so, and if Hamilton still provides sufficient steelmaking capacity to feed into such a rail steel manufacturer. The proximity may compensate for higher manufacturing cost (...i.e. Ontario electricity...), and may be worthwhile given the uptrend.