Geek Chic Rail System Dimensions?
I think the rail designs are pretty clear from this picture off their website.  The exact numbers aren't important if you're building it yourself. 

Assuming this desk is 1/2" thick, then the rail groove entrance is roughly 3/8" thick, and the rail is roughly 1/2" deep.  There is a groove going up behind the rail, that looks to be 1/4" wide, that "locks" the desk into place.  Notice, that most of the force of the desk here is getting absorbed not by the thin piece of the desk going into the groove, but the "side wall."

These are beautiful tables!  Good luck with your project.