Hello - not sure which 312 you have but, like Lionel die cast locos, you usually remove the pilot and trailing trucks, the boiler front, and then thw screw for the compression cylinder assembly. You should then be able to pull the body away from the frame, exposing the engine. No need to remove the screws holding the panel over the the driving wheel gears - but sometimes the truck screws are also go into that plate. For some reason I can't post a link, but go to www.trainrepairparts.com and search for the 312 - you will find a detailed parts diagram of the loco
Some models snap together or may even be glued. Checking with repair shop good idea. Also might find information by searching model # on internet.
Try the train room
Thanks! Your answer is awaiting moderation.