The part of the rail they are talking about is the the channel where the slide rides.
Which is why Flork and Bruce Gray recommend lubing the pistol like this...
The other area to watch out for is the channel itself. This gun has a cracked frame and you can see that the anodizing in the channel of the frame rails has been compromised even in this poor picture. Most Sigs will show some initial wear even with proper lube and will stop. It is a good practice to inspect rails on a Sig each time you clean it. YMMV
