I have all of the possible DCC decoders to put into the spaces on the RS/RSD chassis. The Chassis is 6.9mm wide and the shell is the 'same so no sloppy areas on the side but that is not necessary. The DC Motor/Lightboard is 5.3mm wide, leaving 1.6mm total or o.8mm each side of the board. Most all Decoder wire is o.8-o.6 (for Digitrax and TCS, respectively) and o.54-o.50 for European Decoders. This allows the Decoder under the board to freely route to the needed locations for Track Power from the frame, Motor contacts and the Front and Rear LEDs (plus the Blue + wire). There are 2 Decoder pockets that are both 12.7mm long but the one at the rear is shallow at 2.2mm while the front is 2.92 deep. Since most all of the Decoders are 2.2 or higher, the Decoder needs to go in the front pocket. The DC Motor and Light board can be salvaged so allow easy Motor and Frame power pickups. The RS/RSD series uses a unique PC board so you cannot use the prior drop-in Decoders (Digitrax DZ123Z and TCS-DCC's AZL4 . The possible decoders narrow enough (the critical dimension) and short enough are: Digitrax DZ126T AZL Z2  ESU's LokPilot 5 Micro  Doehler & Haass (Döhler & Häß) PD05A The latter is dang dong smaller, by far. Below are: DZ126T - Z2 - LokPilot 5 u - PD05a Attached will be a series of pix of using the TCS Z2, as it was narrower than the DZ126T, I had some to waste, and I wanted to test the Motor/Light board modifications (which I missed one but no harm), before I wasted time or unknowns get decoders from Europe (through US or Europe direct. Here is the Chassis with attached Motor/Light board and removed board, and modified: The 2 big 'bulbs' are Inductors (a.k.a Coil, Choke) to eliminate Motor noise radiation into an ancient A.M. radio. Remove these to disconnect the Motor from the Frame by clipping them off. They DO NOT go through the board! They were just set in place and solder blobbed to a pad. The 2 large dark rectangles are Diodes that are connected to the track power for LED lighting and directional. Clip them off and then clean up the solder pads. The Blue wire will connect to this. I'll continue, after supper . . ..