This installation is for the N Scale Atlas B23-7 and uses the TCS AMD4 Decoder - performed by TCS
First remove the shell by grasping it on either side (over the "CSX" logo) and rock it back and forth until it slips off of the chassis.
Next remove the fuel tank and the light shield.
Next loosen the screws highlighted above until you are able to spread the frame and lift out the lighting board. NOTE: you do not need to split the frame completely.
Insert the AMD4 into the frame, slip the motor tabs through the slots in the decoder and re-tighten the screws.
Once your locomotive looks like the image above you are ready to replace the shell and run your locomotive!