With your accessories, do you want to be able to turn them all off or on, or do you want control over which one is on at a given time?
If you simply want to have them off or on, it is pretty simple. Run a wire from the fixed post on the transformer to a switch (as others mentioned with track power). Then take the power wires for the accessories, and connect them to the other side of the switch (I like the idea someone else had, buy a switch that has wires already on it, so with the wires coming from the accessories use connect them with the wire on the switch via a wire nut, really easy). At this point, if you flip the switch on, all the accessories will be on, if you flip it off, they will be off.
If you want to control them separately, you would need to buy a switch for each accessory you have. Again, probably easiest to buy a switch that has wires on it already, and here is how you would wire it:
1)On the transformer, put a wire on the fixed output.
2)Have sections of wire, where the number=# of switches you will be wiring.
3)Connect one end of each wire in step 2 to the wire in step one, using a wire nut to tie them together (you will end up with a single wire off the fixed post "fanned out" to wires on the switches)
4)Connect the power wire from each accessory to the other side of one of the switches in step 3.
So if you had 4 accessories, you would end up with 4 switches, where one side of each switch goes to the fixed lead on the transformer, the other side to one of the accessories, and you end up where your accessories are able to be powered up individually.
A simple SPST (single pole single throw switch) is all you need (basically, a standard light switch is a SPST), Home Depot or Lowes will have various toggle or rocker switches that will work, a 120v rated switch will be fine (any switch you use should be rated well above the 18v a typical transformer puts out).
One thing I don't see anyone mentioning, if you isolate that section via a fiber pin or gap in middle rail, you will need to connect a DCS control wire to that spur track (if it doesn't have one), since the DCS signals go over the middle rail and if you isolate the spur the signal cannot get through.
