I wouldn't agree that it is normal wear, I would say it was made that way. I have had over 20 different Glocks, one G21 that went over 125,000 rounds and while it had some wear, it was on the flat part where the slide contacts the rails. Here is a G17 that has about 100,000 rounds and it has some wear on the rear rails also. However, you might run that G19 to over 250,000 rounds without issues, like the Glock guy said. But it is not the way rails should look. Try posting over at GlockTalk.com and see if this has come up with someone else.
Yes I am on GlockTalk and GlockForum with over 4000 views. Only one guy on GlockTalk has marks like this on three of his rails and he has sent his back to Glock so we'll see. You may be right, it could have been made this way and I just missed it. Its not like me to miss something like this but I may have, I just don't know for sure. I was hoping a lot of other's Glocks had this so I could feel confident that it is a non-issue but so far, not. But, when I work the slide slowly w/out the barrel and spring installed, I can't see anything the could cause this. Also, the cracks don't show on the undersides. Seems to me that cracks developing on the top would be from stress coming down from the top but I would think that in reality the operation of the slide creates stress pulling up on the underside. This leads me to believe that it is a non-issue. However, I still struggle with the fact that so far pretty much nobody's Glocks have this.
