In respect of Property Taxes and Railways in Canada, this report to the City of Toronto a couple of years back provides a good explanation and hard numbers. While it initially focuses on Toronto/Ontario, it includes a jurisdictional scan of how other provinces handle railway taxation.
There is no reference to taxation being different for disused corridors, though this may well be the case.
What is clear is that Ontario uses a model of a fixed rate per acre of land, where the prairies use a model based on tonnage, which would obviously become lighter the less traffic a line sees.