Upgrading from Lionel Starter set

Just use common 12V DC wall warts if you insist on external power for the switches.  There really isn't a good reason to abandon track power for FasTrack switches in a command or LionChief environment as they only draw current for the brief time they're activated(aside from two small indicator LEDs).

Thanks, Rob. So all the stuff about 14V preference for switches and accessories and 18V being best for track (but too much for switches and accessories) is overblown?

You really need to look at the individual power requirements for each item you're planning on adding to the layout, whether it's AC or DC.

My recollection is that Fastrack switches run best at 12-18 volts so, like Rob said, there's not much reason to not run those switches off track power as opposed to external power if you're running command or LC  engines. Conventional is different because you want to have consistent power to the switches from an external source while you are manipulating the throttle voltages up and down for the engine.

As a general rule, a lot of accessories run well at a 14v sweet spot, but not all (Menards' buildings, e.g.), so you you need to consult the manual.