Some have, many have not.
Very much so, this ROW is almost entirely in tact. The only built-over part I'm aware of is the Casino.
That's not to say I'm sold on that as a priority investment. There are countless other choices/priorities many of which have higher ridership potential and few obstacles.
In the case of Niagara, the ROW from the west still has tracks to within a very close distance of the casino.
The greater challenge would lie to the east where the track literally weaves between homes and such; certainly, I can imagine many an objection to relaying the tracks across trail, or through someone's backyard.
On that side of the Falls, the case for an LRT running from the VIA station is probably better, on a different alignment.
But again, there are many investments that would be preferable before getting there.
Edit to add: This is the ROW leading into downtown Niagara from the Welland side:
View attachment 385056
The last white dot at the bottom of the picture is where the tracks stop. The ROW runs ~600M in tact before hitting the Fallsview Casino.
Subsequently, it continues intact for 1.6km where it crosses Victoria Avenue in the heart of Niagara's tourist district.