1drummer,
Your very close in your analysis. The DCS signal actually appears between the center rail and one or both outside rails (depending upon whether the outside rails are electrically connected together by the design of the type of track you're using, or because you've installed a physical wire connection between them).
Your locomotive needs both connections (center rail and at least one outside rail that is connected back to the TIU) in order to receive DCS successfully.
Interrupting the center rail's connection back to the TIU breaks the circuit path and prevents the signal from reaching the locomotive, but so would interrupting the appropriate outside rail, if only one of them is connected back to the TIU; or to both if they both are connected back to the TIU.
So, an SPST switch in the center rail connection of the two rails to break the connection is sufficient. A fuse in this line back to the TIU is a good idea, and will not affect transmission of the DCS signal. A circuit breaker however can. Traditional thermal or magnetic breakers should be fine. Modern electronic breakers, like the PSX-AC, will likely reduce the signal.
Switching both center and outside rails would work too though, even though it's a little overkill.
Mike
