Lionel changed the design of the trailing wheel and drawbar a couple of times over the years. Can you post a pic? Maybe a previous owner messed with yours, or you have the wrong parts for your loco.
That being said, on O27 especially the 5121 and 5122 switches, all bets are off. Backing through the curved leg of a switch, especially when pushing a load might well result in a derailment.