In O-scale we generally enjoy more locomotive shell interior room for our DCC decoder installs than commonly found in the smaller scales. It can be helpful to use miniature connectors to do the electrical plumbing between stuff on the frame and what's inside the shell, so that we can easily separate the two.
Miniature plugs can be procured from Miniatronics and other sources, but they tend to be expensive. Here are some I use, running about 6 cents for a male and female pair. They come in 5 packs of 36 pins each. Click on the image to enlarge. Click a second time to further enlarge.
Female:
https://www.adafruit.com/product/3646Male:
https://www.adafruit.com/product/3647These parts snap apart to any length we desire. However, in doing so it is common to crack and loosen the pin at the end of the strip. Since these pins are so inexpensive, take a small diagonal cutter and cut through the center of the pin just past the end of what you want. Trim the rough end if desired.
Strip each wire short, tin the wire and the pin, then tack solder it parallel to the side of the pin. Insulate with heat shrink tubing (slipped over the wire before soldering it on.)
It can also be very helpful to make polarized connectors so that mistakes of plugging it in backwards can be avoided. If you want a 4 pin connector, start with 5 pins. Then fill the end female cup with solder, and cut off the corresponding male pin. The pair is 0.1" longer, but it is obvious which way the pins go during reassembly. We don't want to accidentally connect a track wire to a decoder output or a headlight LED!
