legacy base one wire?

If your running a "conventional only locomotive" it cannot see the command signal so theres no need to break or disconnect the legacy base connection.

This is why it is "possible" (though not ideal) to run conventional and legacy equipped locos at the same time.

Once the conventional loco is moving i.e. voltage is up at the track a legacy loco can be addressed on the same track and operated.

The drawback is the legacy loco operation is not "ideal", because its is limited by the voltage that the conventional locomotive is running at.

Convetional locos run too fast at 18v and lower voltage of say 10 or 12v results in sub par operating characteristics from a legacy loco.

Yes.  You will then be controlling your engine off the transformer handle.  The power master allows you to control  a conventional engine with your remote.

Bruce

Conventional engines cannot "see" the signal so theres no point in disconnecting it

If you want to flip one toggle switch, make it the power control for the Legacy base, that will insure everything runs in conventional mode.

If you wanted to run a "legacy/TMCC equipped loco conventionally you need to merely unplug the base.

The base must be unplugged in this instance because one side of the signal is transmitted through the ground wiring in the walls of your train room via the ground lug on the wall plug. 

Just try and summarize what people are saying.

1.  If you just want to run a conventional engine, you do not need to remove the 1 wire.  Conventional engines will always run the way their were made even if the one wire is connected.

2.  If you want to run Legacy/TMCC engines in conventional, disconnecting the 1 wire may work but is not fool proof.  The only way to ensure Legacy/TMCC engines run in conventional is to disconnect power from your Legacy base.  

    2a.  As an example to why disconnecting the wire is not always enough I can add my own experience.  I moved apartments and had to reconstruct my small layout.  Upon reconstruction, my ZW-L was acting like it was in command mode and my engines were not always.  The problem was my 1 wire had a break in it.  My base was able to send the legacy signal to my ZW-L and my engines when it was close enough to the base.  As the engines got farther away they acted worse and worse.  After replacing the wire everything worked as expected.  Just proving that a disconnected one wire does not truly remove command control.

Correct jmertz, 1 side of the legacy signal comes from the wire and is broadcast through the outer track rails.  The other side is broadcast through the train room ground wring as I stated above.

If I remember Dale Manquen correctly, its the diffeence between these signals that initates commands to the loco which are picked up via loco antenna.

If the base is plugged in but the wire is disconnected the loco still  goes into command mode"looks" for the other side of the signal in command mode.

If NO signal on either "side" is present a Legacy/TMCC loco automatically drops into conventional.

The base really has to see both sides of the signal to sense it, it's the differential between them that develops the signal.  Of course, even without the base connected to the track, some of that signal can still be sensed, especially if there's a wire connected to the output of the base.  I've had my workbench setup sense the command signal when that base was powered off, it was seeing the track signal from across the room.