David,
The center rail will not prevent any loco from leaving the tracks. What I would suggest (and which I have done on my layout) is to make a retaining system all along the outer edges. While I can't get to the layout now to take a picture it is made exactly like the 3 cable retaining system used on highways to prevent crossovers.
The posts are made from hardwood dowels woth 3 appropriately spaces holes. For the cable I used stainless steel wire rope. I don't now remember the dimension of the dowels or wire rope but can get it if you would like.
My table height is somewhere around 38" with the upper mainlines another 9" above the table making for a long way down to a concrete floor covered with foam pads. Since I put up the retaining system I've had about 4 derailments all of which were grandpa error not paying closer attention to grandson. Nothing hit the floor although one of the derailments hit it so hard it cracked the dowel.
Ed
