The metal plates are not intended to be taken apart, unless you have a major failure. The only thing I can add to this thread is a suggestion about approach when servicing these old trains:
1. start with a little cleaning, and lubrication.Most of the time, that's all that is needed. Lubricate the axles, and the motor bearings.Grease the gears. Clean the commutator, which is the 3 segment copper disk visable on the side of the locomotive, just above the drive wheels. You can turn the drivers by hand, and clean this with a pencil eraser, without even taking the engine apart.
2. run the engine. In most cases, you are "good to go".
3. If you notice any issues, then address them. Every engine is different, many are trouble free, some need a little tweaking, and a very few will need surgical intervention.
I have learned to be cautious when handling these old trains, I had a whistle relay fall apart in my hands when I was trying to "fix" it. So, address what needs fixing, but be careful not to take it further than actually required. Start with the simple things first, and then progress to the more heavy duty repairs only if the problems have not been resolved. These Lionel trains are rugged, and robust, and usually don't need much.
You've gotten great advice on getting it back together, and I know you'll be the 2026 expert when it's done ! This is a really great engine. You, and your son will love it !