The Niagara River Recreation Trail is not really a rail trail (though there was once a rail line here), but I include it in this web site because it serves the same purposes, because it is convenient to visitors coming into Ontario from New York State, and because the Niagara region is a tourist destination itself. I have not taken notes as I have done for rail and towpath trails, and the section near the Falls I have not cycled for many years, so please bear with me if this writeup is not as comprehensive as for the other trails.
I have cycled the Niagara River Recreation Trail between Niagara-on-the-Lake and the Falls. Beyond the Falls the trail goes all the way to Fort Erie and I have cycled most but not all of it.
Between Niagara-on-the-Lake and the Falls the trail is a paved path in the boulevard between the road (Niagara Parkway) and the river. Within the City of Niagara Falls you have to ride along streets. The section between the Falls and Fort Erie is also paved and off-road, but not as scenic.
The path can be congested at times, particularly on summer weekends. It is used by cyclists, roller bladers and walkers. Since it mostly parallels the road you do not get the tranquility of most rail trails.
Never-the-less, the Niagara parkway is a 2-lane, winding road with a low speed limit and a lot of tourists who are looking at the sights. It is very pretty with the boulevard and river on one side, fine homes and a few fruit farms on the other. In a few places, particularly at the Niagara on the Lake end, the boulevard becomes quite wide and the path loses sight of the road.
Niagara-on-the-Lake is a pretty town with shops, fine restaurants, and the Shaw Festival. The trail starts at the south end of town, near Fort George. The distance to the Falls is about 20 to 25 km. The first half is below the escarpment, then you will climb to the upper level where the river is in a gorge. Along the way you will want to stop at various viewpoints, and perhaps visit some of the attractions.
The Butterfly Conservatory at the Botanical Gardens is well worth a visit, but be prepared for crowds on summer weekends.
Thanks to Maarten Heilbron there is a video of this trail.