What Lloyd said! In my case I have a large 200 Amp box with lots of room for breakers located in the garage with the layout room just behind it, so I could simply add breakers for the circuits needed in the layout room. If you need to run to a subpanel, the 100 amp would be a good choice. You never know when you might need the extra space in the box for another circuit.
I have one 20 amp circuit for the ceiling lighting, but have that switched as two circuits so I am not burning lights on the other side of room if I am only working at the bench. The bench lights them selves are run from an unswitched house power circuit.
I have another 20 amp circuit feeding duplex receptacles around the room at locations where I thought would need "house power" which is not switched. I put in more receptacles than the electrical code requires (and also met code spacing) as they are inexpensive and I wanted the power where I thought I would need it for things like soldering tools, portable lights for working under the layout, small power tools and a mini fridge.
I have a third 20 amp circuit that is switched that runs around room to feed duplex receptacles for powering up all the railroad electrical and electronics in addition to the power supplies for the LED light strips that illuminate the lower deck. Again, I put in more receptacles than the code requires.
The two circuits powering receptacle are distinguished by one set being ivory colored and the other white colored. Switches for the overhead lights and the layout power are all located next the entrance door to the room.
Experience has shown the unswitched power circuit has worked out well. However, experience has proved that while I thought I included plenty of switched layout power receptacles, I did not include near enough and have had to resort to "outlet expanders" and power strips. It is amazing how the number of things on a layout that need electricity keeps growing. I should have put in quad boxes for each of the layout receptacle locations with two duplex receptacles each. The other item to be aware of is that some wall warts want to occupy more space than just the receptacle they are plugged into, preventing use of other receptacles adjacent to it.
I also have sheet of load for load calculations of each circuit which was really important for lighting in the days of incandescent lighting. Not needed with LEDs having replaced at the power hungry lighting. The other circuit loads are no where near the code 80% of breaker ampacity planning figure.
On3 Durlin Branch