Brent, I have worked on many of these Lionel Legacy GP35 engines with 28 in my collection. The GP35 is an easy engine to open. Closing the engine requires guiding the battery cable so it does not pinch between the chassis and body shell, and avoiding pulling the antenna wire out of the board. I slide the body shell on, front end first to get it to align around the Cab interior. Check that no wire gets run over the top of the smoke unit.
I have pushed the board connections together on an occasion or two. (I have switched non-powered body shells onto eight GN GP35 Legacy chassis that I bought from a dealer, in order to make a third powered with sound & smoke for a three engine consist in several road names.)
If there is an engine to attempt to open, the GP35 is one of the simplest. Put the body panhead screws in a small dish so you don't lose them. The body shell will lift up and be laid on its side with all wires connected between chassis components and the engine shell.
Check that the antenna wire coming from the roof metal plate is plugged into a board. It is the shortest of the interior wires and could get pulled out. You will see the two boards Gunrunner John pictured. Push then gently together and see if they get closer together. There are two rows of pins that should plug into each other.
THE ORIGINAL TRACTION TIRES on the GP35 and some F3s were too fat/thick and raised the wheel high enough off the rail that the wheel flange could not guide the trucks through switches and around curves. The outside edge of these too thick traction tires will wear and bevel themselves causing the engine to jump the track. The correct traction tire is Lionel part number #6208815206. I just replaced the traction tires on a 6-38875 B&O GP9 #6456 that wore the edges of the too fat traction tires and jumped a switch.
Maintenance on traction tires and smoke units will be a regular job over the lifetime of your engine.
Sincerely, John Rowlen