isolate the middle rail in the track you want to switch off (at both ends)
Connect one wire to that middle rail and take that to the M84
Feed middle rail power (from same booster/CU) to the M84 port that can be switched to the one used in previous step.

The ground need not be switched.

But why switch power off, to save electricity? The whole idea behind digital is so that you don't have to isolate sections of track.

Marklins simple diagram for this is at the bottom right of page 14 of the manual.

The wiring to the #4 port pair is as Dale describes with the source power shown as being picked up from the "Track" and wired into the middle of the 3 socket terminal block
"Track" means live power and may equally be represented by showing the red (B) output of a controller/booster

"Stopping area" represent the piece of track you wish to control power to, and is wired from the 4Green port.


What you should be considering is which m84 to buy and how to configure it.
By default the m84 is configued with 4 port pairs to operate as SPDT switch with the power being switched to EITHER the red (left) or green (right).

When used of a dead section switch, the "red" contact of a port pair in the above scenario becomes a waste of space.
That is until you appreciate that the inside of the m84 it has TWO relays, one for green and one for red and that it is purely by configuration that they appear to operate with only one at a time - i.e. like the SPDT

With the m84.2 (60842) we now have the ability to address 8 ports individually (rather than 4 port pairs). This immediately changes the financial consideration by halving the effective cost per port, as well as reducing space requirements.

The only limitation is that the middle socket of each port pair is common.
Thus for e.g. you could have ...
- port pair 1 switching 2 digital track sections
- port pair 2 switching 16V AC to 2 accessories
- port pair 3 switching 12V DC to 2 accessories
- port pair 4 switching the sound ouput of your stereo to 2 different speaker (either turned on together or individually - simulated train moving left to right)

For a hidden fiddle yard, this means a single m84.2 can control 8 yards/roads each with its own address.

There is a workable solution with the m84.1 (60841), as long as you a prepared to only need one port live at a time.
The m84 operate by processing a command in two steps
1) turn off the "mate(s)" port
2) turn on the main port
This is normally seen when you press "#1 green" where (1) the red relay is turned off and (2) the green relay is turned on.
What determines the "mate(s)" IDs is a mask, and it is here that the mask can be edited, for e.g. , such that when you press "#1 green" where (1) the relays for 1R,2G,2R,3G,3R,4G,4R are turned off and then (2) the green relay is turned on.

While this may emulate 8 track fiddle yard control, you might consider planning for 7 yards and leaving the 8th to mean "all off"

ref my post @ https://www.marklin-user...s--CS3--m8--2#post447390