I just googled 'model train shelves' and, of course, got waaaay more 'hits' (52 x 10^6) than I can process... (lessee, at pi x 10^7 seconds/year, that just over a year at one hit per second, not counting bathroom breaks....) A couple of things I did see on the first page was a) lots of pictures of various incarnations of train shelves (if you click on one of the pictures you'll get many screen-fulls - err - screens-full?) and b) there do seem to be some Al shelves sized for O-gauge that didn't have the 'middle rail'. I have no idea whether they are any good, etc. What I was looking for was an example of what I think of as a stair-step shelf, where there is room for two rows of trains, with the back row on a step up so it can still be seen. It wouldn't have to be a full step (meaning the full height of the train in front) - it could be half that or ... I didn't really see a good example of what is floating around in my head - but as an earlier poster noted, since you aren't running these trains, you don't have to have much clearance at all between the two rows of trains. The advantage of this - especially using half-height back steps - is that one can get ~4 rows of trains in the same vertical space as ~ 3 single-depth rows (YMMV).
Since I do live in earthquake country (assuming we don't burn up first), having some way to prevent trains from peeling off the shelf when the 'moderate one' hits seems like a good idea. The pegs are an interesting thought. At this point, I'm still inclined to have some sort of glass or clear plastic front - perhaps sliding in dados in the front top and bottom of each shelf.