The rail you see is actually about 60 lb. I say "about" because it's rail from an abandoned standard gauge siding.
We choose 56 because a) it's light enough to manhandle with only a few people, and b) it's heavy enough that, as Glenn says, it doesn't "flop around" once it's spiked down. Alas, with the price of steel going up it's getting more expensive to buy it. All you see in the photo was donated from Aberdeen & Rockfish RR, in North Carolina.
We have some 25 or 30 pound rail, which came from the Ramsdell farm, which originated from the Kennebec Central's Randolph yard. We use a few pieces of that in our shop and shop leads (but it flops worse than the 33# we use).