Taking Apart a Model

[prr22]

Apologies if this question has already been asked and answered before, but may I please ask for some assistance in disassembling my Thomas and Friends model?  I've gotten to the point where I removed the face plate, can separate the chassis from the body, and can wiggle the cab roof.  I don't want to remove the wrong screw and have it explode into a million pieces, so does anyone know what the next right step is?

...I'm experimenting fitting a DCC decoder into T&F locos.  Trying James and Thomas first.  At a train show, one of the members from my club brought a Hornby Thomas with DCC to our modular layout, but with no couplers LOL.  On the spot, we retrofitted a coupler off a Bachmann Thomas, and let's just say it didn't go too well.  So, I decided to take the more planned out route, and use Bachmann.