We also had this problem, especialy with babies from ribbons. They crawl up between the overhanging part and then they are on the glass in the rail part. The problem is you don't see them, but you notice they're there. But lucky we never had any damage to a garter by that.
Now we use some kind of brush-thing on every slidingdoor.
By placing this on the behind glass (b), the front glass (a) will slide directly alongside the brush, so there is nolonger a gap and the babies can not crawl up anymore.
