I got a PM from someone trying to add a new block of track to an existing layout, using two controllers. I'll post an answer here for the benefit of others:
Hey jack,
I just started a new section on my railroad and i am not sure how to wire it for block wiring. It will be a 2 train operation and i had it done 1 time before but i lost my sheet on how to do it so do you think maybe you could give me some insight on how to wire it up .
First, there is the consideration of whether or not you are using common rail wiring. I will assume for now that you are not.
So, you have two wires (leads) coming from the rails on your new block of track, and you want to connect these to the four wires from your controllers (two wires from each controller, aka cab). What you need is a dual-position dual-throw toggle switch (do a Google image search on "dual-position dual-throw" if you want to see some). these are also called dual-position dual-pole toggles. The toggle has six terminals on the bottom, which are in two rows (hence dual throw) of three. In each row, the center terminal is the one that gets switched to one of the other two. So the center terminal is the one that goes to your track block. Decide which row of three is to be positive and which negative, and wire the center terminals to your track block, and each of the other pairs to one of the controllers. Now, when you flip the toggle, it will connect the track to one controller or the other. If you get the ON-OFF-ON toggle, it will also have a "dead" middle position, which can be useful if you want to have no power to the block.
If you are using common rail, it is simpler, since you need only a single-throw dual-position switch. This connects the powered rail to one or the other of the positive terminals on the controllers. The negative terminals on the controllers get tied together with all the common rail leads on the layout. Again, wire the center terminal to the block, and the other terminals to the controllers.
Now, here's a cool enhancement to consider: If you have common rail, you can use a dual-pos dual-throw switch to good advantage. Use the one throw for the track power, and use the other to switch accessory power to a pair of indicator lights, showing which of the power packs is supplying power to your block. Using lights of different colors (LEDs are great for this) makes it really spiff. The power comes from the accessory terminal, or other power supply (I use a simple 12v supply), and gets wired to the center terminal. The two indicator lights get attached to the outer terminals, and then to ground.
I hope that gets you where you want to be.
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Now, for the more adventurous, here's what I have done: I have four controllers switched to eleven blocks. That is, each block can run from any one of the four controllers. For this, I needed a rotary switch, so that I get four positions. In the end, I got a 6-position rotary, so that I could include a "dead" position at each end of the switch, where there would be no power to the block. Since I am using common rail, I could have used a single-throw 6-pos rotary switch, but RadioShack has these nice dual-throw 6-pos ones that will serve well, and will provide another service: I used the second throw to select 12v power to one of four LEDs (four different colors), corresponding to the four power packs. So now I have a red, blue, green, and yellow controller, and when a given block switches to one of them, it also switches on the appropriate LED to show which controller is powering each block. This is very very nifty, since I can see at a glance which controllers are powering which parts of the layout. So, I end up with 11 rotary switches on the control panel, each with a set of four indicator LEDs. And it works like a dream! The wiring was tedious, but so worth it.
BTW, I have entered this layout in MRR's track plan contest. Wish me luck!