My grandfather, Lee Wilcke, made the bed for me and he made it for a twin mattress I used to own. That mattress was a little bit thinner than the one we got for Harrison (Gpa Greg and Gma Pam - thank you!). The rail is only about 2 inches above the mattress but Harrison has always been a bed sleeper so we are not really concerned that he will fall out, but I guess you never know.
I thought this would also but a damper on potty training but so far so good. He climbs down just fine in the middle of the night and climbs right back up to go to bed. With any luck we should be out of over night pull-ups before he turns 4. Yipeeeeee!