Thanks to all of you who have replied. I will try your suggestions and let you know how it works out.

The engine is #3781 AT&SF part number 41-580-02. I don't know how old, but certainly pre-DCC. The electric pickup is with brushes on all 4 wheels of the trailing truck of the engine. The tender seems completely independent and has no electrical connection to the engine. I believe I have to remove the shell of the engine and hope that there is room above the diecast chassis for me to place and wire in the decoder, which is a wire harness type, not a plug in to a socket.

I am recently returned to model railroading after a 15 year absence. I decided to start building a new layout with DCC control. At the same time, my brother gave me an older Bachmann 4-8-4 without DCC. I purchased a decoder for it, but, since I have never had this type of engine, I don't know how to remove the shell. I tried various things without success and I am looking for advice on how to disassemble this.