Credit: Again, thanks to Kraftrains for their hardware, it's great, check them out, and also thanks to Chris33333 for the base parts.

Overview:
So this is an adaptation and elongation of Chris33333's Combine for standard gauge HO scale. However, this is significantly set apart from it as it is designed to function as a real mail car would. it has 4 arms that swing out the doors that allow you to deliver mail bags (not included) to trackside stations (not included).

Assembly:
Again, I'm bad at explaining but what you wanna do is take one of the little arms, and hold it so the longer red bit (see photo) is sticking up, the orange bit sicks away from the car, and the shorter green bit points away from the door. There are two types, so make sure you've got the right one for the side of the door you're doing. once you have the red shaft into the hole as shown, take a soldering iron or a hot paperclip (heated up in a candle or whatever, use pliers to handle) and flatten out the top of the red bit till it holds on when you let go and swing it around. Once all four arms are in position, slide the door onto its rail inside. Then you can take the body and roof and glue them together. Then, take one of the trucks and pins, put the pin in the truck and align with the chassis and hammer it in. Afterwards, fit couplers. Then you can put handrails in (I use paperclips, bc they're cheap and handy) and glue the chassis to the body and finally put wheels on.

Operation:
To use it, slide the door open on the side of the delivery, pull out the arm facing away from the direction of delivery, and place the delivery on the arm. Then, couple up to a train, and go about your service or whatever. However, when you get to the delivery's destination, slow down as to not damage anything and the delivery can slip onto whatever hook you have for drop-off. Make sure your drop-off hook is the correct height or it will derail the car. you can also push the arm into the car and have the door open still with the parcel still on the arm if you're not coming up on the station or have multiple dropoffs.

Thanks!