New build, figuring out how to wire

Updated the diagram (minus the turntable and whiskers) to show individual track blocks following the guidance in the DCS O Gauge Companion book:

• Attempt to limit the number of track blocks or sidings from a single TIU channel to no more than 12 - 15
• Limit track blocks to no more than 11 or 12 track sections where sections, long or short, all count. If sections are all short, consider soldering connections between sections to make longer ones. Regardless, track blocks should not exceed 100 feet in length

According to this I should plan to use two TIU channels, so I'm assuming I'll need two PH180 transformers.

I'm attempting to figure out the wiring as much as possible prior to attaching the baseboard. If there are two power districts via two TIU channels, then does it make sense to insulate the outer rail common between power districts if not between blocks?

  If your running modern trains which I assume you are as your using the Fixed channels. They really don’t draw a lot of amps. The biggest offender is probably the smoke units more so than the engine. Most lighting is all LED’s now. If you feel one transformer will do the job. You can feed both fixed channels by jumpering the inputs with wires. The Lionel 180 features a good fast acting breaker. But I would probably protect each channel individually.

As far as the outside rails. No need to isolate them. Just do the center rails for your blocks.

A couple of thoughts from my experience in running DCS and Legacy. I would recommend using all 4 channels of the TIU; this will help keep a good DCS signal. I would have your yard and sidings on a separate TIU channel that you run through toggle switches so you can turn off power to each.  I know you mentioned using BPC for sidings but that can get pretty tedious which is why using toggles on a control board is what I suggest. ( Note I do run all of my switches through AIUs connected to DCS; not tortoise switches) I power my layout with two Z4000s using both handles on each; based on that I would use four PH180s. You also need to think about what buildings and accessories you want and how to power those. As mentioned by Dave, engines don't really use a lot of current, but some of the older passenger cars can use quite a bit (unless converted to LED). I don't have a turntable but thinking you would run that area on a separate TIU channel. Then two TIU channels for your two loops.  As far as a ground loop for Legacy, I did not have any issues until I put a second level, so I would start with just hooking up Legacy to see how things work. Adding an earth ground wire is easy enough to do after track down and tested.

Thanks @dave c. and @Kenjr for your thoughts.

I made a diagram of using two transformers and two TIU outputs powering half of the layout blocks.

Whether or not I end up using more than two TIU channels, is there anything I need to think about when using more than one transformer? The hot center rail will be isolated between blocks and thus power districts but does the common need to be isolated between power districts as well? Do I need to connect the two transformers somewhere?

All transformers on  a layout must be phased.  Lots of threads on this forum about how to do it.

Common should all be tied together. I jumper all the black posts on the TIU output side; that is where I connect the wire for Legacy.

The transformer output red/black connect to the red/black post on the TIU input; one brick per channel. You can use one brick to power more than one channel by jumpering the input, but don't hook up two transformers to the same input channel.