I bought a Minitrix depressed center flat car with a transformer load many years ago, 40 years maybe. It's a German prototype but I think it looks good on my layout. I was looking to see if I could replace the Rapido couplers with some type of knuckle couplers, Micro Trains preferably. I can't seem to get the coupler pocket open. I don't know if I need to apply more pressure to pop off the cover or if there's some other way to get it off. Does anyone have any experience doing this?