usually there's screws on the underside of the fuel tank that fasten the shell to the chassis. the cab is usaully held in by tabs. I'm honestly not familar with that particular locomotive, how ever, that is how it is with some of my engines. as a general rule of thumb, never use force to remove a piece unless absolutely necessary.
just my
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Hey, I have a couple of these, If you have a model from within the last 10 years, all you will have to do is plug in a decoder, unless you want to hard wire something that will give you more functions.
If you haven's already gotten the body off, the first thing you need to do is remove the couplers. It's much easier to take the shell apart once it's of the frame. Look at where the tabs are. I broke a couple of mine, but it's such a bad idea I found another way to hold it in place.
If you have an older model that was made in either austria or in japan it should be very easy. Again remove the couplers. Then the sides have pegs on the side peek through the body. Just lift outward and pull up. If by the way the model is from japan, congrats you have a pre-Kato era model. The japanese version of Atlas broke off into Kato back then.