but I would like to incorporate the Katy Trail, and others like it, so the perfect tire needs to be able to handle some off road conditions
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I have done a lot of week long ultra light tours and have had pretty good luck with 700x32 Panaracer T-Serv folding tires which I routinely inflate at 100 psi. I a considering going up to 35-40 mm due to the extra load I will be carrying.
I have ridden parts of the Katy Trail with my regular road bike and 23/25mm tires. It has pea gravel and fines, well packed (except after snow). Rolling resistance increases somewhat, but I wouldn't call it off-road. It is also mostly flat. A 10' climb over a dike is considered a mountain.
I assume other unpaved rails projects would be similar.
I have a 700x32 Panaracer on my cargo bike. It is a nice tire, and so far seems to be holding without problems. It certainly is much heavier than the 25mm Panaracers. I think the 32 has a reflective whitewall that the 25's also lack, but it seems like a nice feature. My 25mm Marathon also has the reflective whitewall.
My only Marathon is a 25, and it is heavy. Are the 32's and 35's heavier?
Personally I like the narrower tires, but with your total weight, maybe a 28, but the 32's sound like a good choice.
Anyway, a 32mm Panaracer would be a good choice.