Power switches for dual-mode layout.

palallin posted:

...  If I can gang one SPDT and two DPDT knife switches (or toggle switches) together so that they all throw at the same time, I can make this work, but I can't find any "modular" switches that can be ganged  in that manner.

If I understand your question, you're asking if there is a widget akin to "handle ties" used on panel circuit breakers to mechanically interconnect adjacent breakers?  In other words, something that mechanically interconnects 3 adjacent toggle switches so all 3 handles flip at once?

Such a mechanical connection would be one way to solve the problem, yes!

How about a 5-pole rotary switch?

How about a switch controlling the coil of a multi contact relay?

3PDT switch? Or I was also going to say relay, but it's suggested just above already.

I’m a member of a club which has 12v DC and 19v AC for DCC, wired to the same test loop. The security was much discussed and the solution, since the transformers are different units, is that we have a plugboard attached to the baseboard.

On that plugboard, two sockets - a coaxial socket for DC track power, and a proprietary socket for DCC. Neither are mutually compatible. 

To switch over, we turn off the power at the wall, remove one socket, plug in the other and turn the power back on to the relevant transformer. Simple and unambiguous. 

It does occur to me that if you want to switch between 2-rail and 3-rail, you will need to pay attention to endure that (1) you have no “common return” connections anywhere, linking both outside rails (2) you have no reversing loops, because they require special arrangements in 2-rail. 

RoyBoy posted:

How about a switch controlling the coil of a multi contact relay?

That's the ticket. Do  a search on eBay for "Fuji Electric SJ-0G SJ12AG 24 VDC Coil Reversing Motor Starter SJ-OG SJ0G".

This is a reversing motor starter. Wire the DC on one side, the AC to the other. They are mechanically interlocked meaning you cannot have both energized at the same time. Control it with a selector switch. Use a NC contact to energize one coil, a NO contact to energize the other.

Cheap, easy, done.

Thanks for the ideas, gents.  A 5PDT would work--need an extra pole to eliminate the common return needed for 3-rail but deadly to 2-rail.  No reversing:  just a simple oval.  Gilly's idea may be the simplest ticket. . . .

If the original question is about a widget that allows mechanical "ganging" of multiple switches, that's one thing.  But if it's an electrical question of how to steer/select how 3 rails go to 2 transformers, then why doesn't a 3-pole 3PDT as rtr12 suggests work?

Pole 1: outer rail #1: toggle selects AC- or DC-

Pole 2: center rail: toggle selects AC+ or nowhere

Pole 3: outer rail #2: toggle selects AC- or DC+

in which case there are modestly priced 3PDT toggle switches with 10 Amp AC capability:

And the center-off version would connect the 3 rails to neither transformer which might be useful for troubleshooting or whatever.

Well, thanks to all this information, I have a 3PDT Center-off switch on order. 

I appreciate all the help!

You do not want the 2 outer rails connected anywhere since the other mode of operation is 2-rail DC.

I failed to read the first post correctly, my very bad.  I mistakenly read "2 rail"  and "3 rail" as two separate tracks.  I can do dumb quite well.  A  3PDT is exactly what is needed.

Betcha I can do dumb better, Tom:  I was *really* over-thinking this

Palallin,  If I had been smart and read the post accurately I would have given you a 4PDT toggle.