Not sure why it's listed as Code 55; I just measured it, and it's actually code 58. And their flex is code 60. Go figure.
Just for reference, the heaviest rail ever used in North America was 155 lb. (by the PRR), which in Z would be code 36; 100 lb. rail would be code 26.
Also for reference: Z scale flanges range between .018 and .022, so you've got room to pile on a lot of dirt.