The installation is for HO Scale Bachmann Niagra 4-8-4. It uses TCS MC2 digital decoder and was performed by a TCS Customer.
In order to get the shell off you twist the rear trucks of the locomotive to the side and there is a stainless steel screw that you have to take out. Be careful not to take out the screw that holds the connecter for the tender on, but the other screw that holds the shell on. Once the screw is out the shell slides up in the rear, then to the front, and then it'll pop off.
Once the shell is off you have to separate the two sides of the frame. There are three screws holding the two sides of the frame together, but at this point only two are visible. Take these two screws out, and if it is then possible, remove the smoke unit and the light bulb, so that they are out of the way. In this installation we will be leaving the smoke unit as DC, so one only has to add a few drops to the smoke producer to make it run.
Also unscrew the two black wires that come up from the rails. DO NOT CLIP THESE WIRES OR THE TABS. You will be attaching the red and black wires to these tabs later on.
Then you have to remove the two screws from the gear box on the bottom of the locomotive. Once these screws are out, flex the front of the chassis away from the frame and then pull it away from the frame, the chassis will come off of the frame, but be careful not to break any of the detailing. The rear of the chassis comes off in the same manner.
Once the chassis is removed you can pull the wheels out. The third screw holding the frame together is behind the fourth wheel and should be removed at this time.
The last thing that needs to be done before you can take the two sides of the frame apart is that one of the valve boxes has to be slid off of it's side of the frame. One of the valve boxes on our loco was glued on, so make sure that you check before just pulling one off, but even the one that wasn't glued on still had some sort of adhesive in it holding it together, so it does come off hard.
Now that the frame is apart, lay the one piece to the side. Take out the motor and clip the leads short enough so that they aren't going down the side of the motor, but leave them long enough to solder a wire to them.
Also notice that the motor is marked with an arrow pointing to which side is down. The orange wire gets attached to the bottom side of the motor and the gray wire gets attached to the top side. You may be able to use a T1 for this installation, but we didn't want to wire one up and then find that the shell didn't fit back on, so we went the easy way and used an MC2. An M1 would also work, and if you're daring you could try a T1, but you might end up tearing out the T1 and replacing it with an MC2 if the T1's too big.
Once the motor is wired up use some kapton tape, and tape all the way around the motor at least once, but no more than twice. This not only isolates the motor tabs from anything else, it also holds the orange wire in place, for coming up around the motor. Now the frame can go back together the way it came apart and all that is left is the light and the power pickups. For the light we used a new 12v bulb that had wires coming off of it instead of uninsulated wire leads, so that we wouldn't have to worry about isolating the bulb from the frame. One side of the bulb gets hooked to the white lead and the other side gets hooked to the blue lead. Be sure to shrink wrap or tape these connections.
Then hook the red wire up to the tab coming from the right rail and the black wire to the tab coming from the left rail and you should be all ready to go. Screw the red and black tabs back in, put the shell back on and off you go.