It's regular old hot rolled 1.5" square .120 wall tube that I bought from the local steel yard(Santa Rita steel in Tucson, AZ.) I couldn't find 4130/4140 tubing in that dimension, so I went with what I could find. I bought 3 feet of it and chopped it in half, I'll trim off the excess as I finish it up.
Onlinemetals.com is an online source for this stuff.
Order Mild Steel A36 Square Tube in Small Quantities at OnlineMetals.com
Just be warned that this stuff is heavily stressed when rolled, so when you make your first cut, it will pop and spread open at the cut. Nothing a bit of vise action can't fix later. The only real up side to using the A36 is that it really files easily if you don't have a mill to cut the rails.