The first step in preparing to lube the internal components is removing the shell from the frame.  To start, with many locomotives, includ

Next, gently pull the shell off the frame, grasping the lower frame,  which was covered by the fuel tank, in one hand, and the body in the other. Sometimes it may help to pry the body off the frame at one end with the blade of an Exacto knife.  Be gentle!

Other engines, manufacturers can be very different, especially older KATO.

Continue to pull up on the shell, and it should lift completely off the frame.