Dual mode DC and DCC

People ask about wiring a layout for both DC and DCC. Don't do it.

One idea is to have a big switch that selects from a DC cab or the DCC control station. A dpdt switch can select between ONE DC pack which can run one loco, and one DCC booster that can run many locos.

How will you handle multiple DC packs with one dpdt switch? If you are proposing
one dpdt switch per block, people do this but it has to be done carefully to avoid "letting the smoke out" of equipment. I don't quite understand the damage that can be caused (the DCC standard actually defines a DC-powered block to mean a stop-block) but folk do warn against mixing them up.

When i was pondering doing this I ended up with a design where the Big Switch actually controled a relay on every block that made sure ALL blocks were on DC or ALL blocks were on DCC.

DCC can draw a a lot more power into one block than DC (think three sound-equipped locos) so it generally needs heavier wire, especially on long lengths. If you have one set of feeders they have to be heavy enough for DCC but provided with all the block on/off and cab-select features required by DC. So an exisiting DC layout may need rewiring.

If you ever want block detection you'll need different devices on the block feeders of each type, so how do you manage that? Separate wiring might help?

If you have separate track feeds for DC and DCC this doubles the effort and amount of wire under the layout.
And I'd be worried they may interfere with each other. Where will they meet up? Actually on the rail (separate feeders for both) or will they share feeders?

DCC can have devices out on the layout attached across the DCC power feeds: local stationary decoders controlling turnouts etc. Put DC on the rails which feeds back thru the DCC wiring to them....

And how do you guarantee that the DCC and the DC network never go live together?

I'm betting the only folk who have been successful with dual-mode layouts have simple systems with nothing but throttles and trains. the moment you want block detection (as compared to spot detection using infrared etc), signalling, transponding, autoreversing, power sector protection, turnout control, turntable driving, or a host of other features on your layout, then dual mode will make them difficult or impossible, regardless of how you wire it.

Just use address 0 on DCC to run the occasional DC loco, then let go and move on into the bright new world of DCC.