This installation is for the N Scale Atlas C628 and uses a TCS AMD4 Decoder - performed by TCS
First remove the shell by gripping it near the center of the locomotive and gently rocking it back and forth until it slips free.
Next, loosen the screws highlighted in red below just enough that you can slip the original lighting board from the chassis.
Your locomotive should look like the image below after the lighting board is removed.
Next install the AMD4 decoder into the chassis in the same orientation as the original lighting board.
NOTE: It is best to slip the motor tabs through the slits in the decoder as shown in the image below. The ensures both a strong connection, and proper isolation.
The image below shows the motor tabs slipped through the slots in the decoder with a bit more detail.
Once the AMD4 is mounted in the frame you can tighten the two screws back up, replace the shell, and you are ready to run!