power pack for accessories only - Model Railroader Magazine

Well, it would depend on what "accessories" you are planning to use.

I have a lot of structure lighting, which I was running from an ancient (like Truman Era) Lionel transformer.  I had maxed it out, and every time I added another bulb everything got just a bit dimmer.  So, I went to a local electronics place and picked up a 12 volt DC, 1.5 amp wall wart.  This cost all of $20, and my lights are all much brighter now.

While replacing the old transformer, I discovered that I was also powering my Walthers traffic light controller.  To my surprise, that thing wants 19 volts!  It's an odd voltage indeed, so I've still got Mr. Truman's Lionel running, just for that.

There is really no "standard" voltage for things.  I have some Miller Engineering electroluminescent signs, which take 3 or 4.5 volts.  I found an old wall wart for that.  The Kadee electromagnet uncoupler draws a whole bunch of current, so I dug into that wall wart box to give that its own power supply, too.

If most of your need is for structure lights, I'd go with a 12 volt supply.  Remember to look for bulbs in the 12-16 volt range to light your buildings, and you should be fine with an amp or so.