There are some advantages to having things centrally located. And, what works best for you will depend somewhat on your hardware choices. I prefer having most of my electronics in a few easy to reach locations, and minimal wiring distributed under my layout.
Three parallel buses use more wire, but the loads are less and the voltage drops for the same gauge wire will be reduced. The current in the nearest section of a single bus would on average be three times what's in each of three parallel ones, and the voltage drop for the same wire size also tripled. Larger layouts benefit more from saving multiple buses, but the longer runs mean the wires need to be heavier gauge, and then multiple summed currents mean it needs to be even heavier. What is the maximum voltage drop you will accept in your track power? What's the maximum current your system would have in that single bus, and how long will it be? Look up the wire resistance, work out the total resistance out and back and the resulting voltage drop at the maximum design operating current. You may be shocked at the wire gauge you should use.
If there's anything that needs to be sensed and reported, it's easier to do that in a central location than bringing back signal wires from all over the layout. And, "signal" type lower currents and voltages are more easily corrupted by higher power nearby circuits, so it's a very good idea to separate signal wiring from track power wiring, not route them in the same cable or even two parallel cables a few inches apart.
My track wiring was begun 40 years ago, for DC power and a computer interface. My layout measured 10 x 20 at the time. It has since grown a foot or two in each direction. But, it's still "medium size", although a central aisle means wires run about 30 feet in two opposite directions to the farthest points. The bulk of my railroad's electronics are centralized near the main control panel. I didn't want to have a bunch of complex electronics scattered all over under the layout.
All my blocks have current detection, on cards that also have relays for switching track power. This is still needed for detection with DCC, and the relays are handy for for locating and isolating short circuits. My cards have the relays and detectors for four blocks on each card, and the cards are mounted side by side in a rack. The railroad control computer is right below them, all connected through a 50 conductor ribbon cable that acts as the backplane for the cards.
It works for me. If the layout were much larger, I'd probably have multiple nodes like this one, and have them communicating wirelessly now, or over ethernet cabling. In summary, I'd say it's a system design decision. Not many of us probably think about our railroads' wiring and electronics that way, but it's a good analysis tool..
Don Weigt
Connecticut
