Re: Wiring help needed
Tim, If the layout is indeed that of https://www.trainsetsonly.com/page/TSO/PROD/150-HO16 as Jay has suggested then...
1. There are no reversing loops so no DCC polarity reverser is needed.
2, You might want to isolate one or more sections for either electronic circuit breaker or track occupancy detectors for signaling, etc. Passing sidings are a good location for this.
3. The easiest way to do #2 is to provide plastic isolating joiners on all 4 rails of the frog end of mainline turnouts/track switches.
4. Doing #3 forces you to provide individual DCC power feeders to all isolated sections of track. Drop these feeders down under the layout to connect to either a common DCC bus following the general flow of track... OR as each track section is fed through individual occupancy sensors or circuit breakers added now or later to suit your plans.
5. Put your DCC command station/booster near the middle of the layout "L" and fan out the DCC track power bus in both directions to flow under the layout, but do not connect it up into a continuous loop. Use #18 or larger wire for the DCC main bus... in twisted pair or 'zip' cord.
6. If you plan on using accessory decoders make provisions for a separate DCC feeder sub-bus to power those units. In that case provide an electronic CB (PSX is the suggested brand) for the main DCC bus feeding the track and wire up the accessory sub-bus directly from the booster output.