Common Return (or Common Rail) is a method of wiring used in single or multi-cab analog layouts. It saves wiring by by requiring that one rail is electrically connected throughout the entire layout to a common ground. Then only the other rail is ever used to isolate block sections or switch between cabs. For isolation purposes, this requires only one SPST switch on the control panel and one additional wire for each section.
The term is not used in DCC layouts because for small layouts with one booster both rails are common, not just the return. In larger layouts with multiple boosters or power districts, there is no common rail, all power districts must be fully isolated with gaps in both rails.
When multiple cabs are used, each cab must have idependent transformer windings. Products with multiple cabs in one, such as the MRC280 Dual Throttle Controller allow common return wiring.
NMRA Data Sheet D7m does not recommend using a common return with command control systems. Use with block occupancy systems is not recommended either, for reliability and easier implementation.
Common Return Between Booster Districts
If the layout uses more than one booster, all the boosters should be connected together with a heavy wire on their ground or common terminal. This will establish a common reference between all components, as well as a current return path between booster districts. This will allow the autoreverse function to work faster as well.