I've been on various rail and canal trails on a Trek 520 and on a LHT. For those with a compacted crushed stone surface pretty much any tire is fine though wider ones are better if the trail is wet. This as the wet compacted crushed stone becomes soft and you will sink in a bit and leave tracks. For the trails that are more clay, roots and rocks than a graded surface wider tires with a bit more aggressive tread pattern work better. This as the more aggressive tread helps you keep from sliding on the wet clay and roots.
I'll be doing the Erie Canalway from Buffalo to Albany next month and opted to put Schwalbe Marathon Mondial 700X40 tires on my LHT. This as prior experience on the wet Canalway told me that wider is better. I took the bike down a rough rail trail yesterday and was pleased with the tires' performance. A heavy thunderstorm had come through and the trail was saturated, had a lot of standing water, and was soft. I left tracks but despite the trail conditions I had no problems riding and no slipping. To my delight I have also found those 700X40 tires to be great on the local roads that are in poor condition.
On my past two long road tours I had 700X32 and 700X35 tires on the bikes. They were OK on the paved roads but I would have preferred something wider and more aggressive when I took my off-road detours.
All that said, I'm sure that the combined weight of the bicycle, gear, and rider has a lot of influence on how well tires work out on any particular trail.
