AC/DC adapter

You could power most any accessory, especially building lights. For less than $2 each, a DC-DC Buck Convertor with each accessory would let you dial in the optimum voltage. If you want to get into leds, you could easily power over 150 of them. I use a computer power supply with 12 vdc to power all my buildings and also crossing signals and gates (MTH). I even use it for the smoke in two K-Line diners and the MTH brewery.  Lots of possibilities.

Laptop adapters are perfect for powering the aux port on the MTH TIU.

Elaborating a bit on earlier comments, here's a recycled photo from an earlier OGR thread.  Depending on the laptop brand, it might use a barrel/coax connector.  For about $1 (free shipping) on eBay you can get an adapter to convert the connector to screw-terminals which generally makes connections easier - so you don't have to cut/splice the plug from the cable.

As for the DC-to-DC converters, for less than $3 (free shipping) you can also get a version with a built-in DC voltmeter so that you can easily set the output voltage.  While many guys seem to have DC meters, many guys don't (or the battery is dead, or it broke, or fill-in-the-blank).

19V x 3.4A = 65 WATTS.  That's a lot of power for accessories.  Note that you can power multiple DC-to-DC converters from your single charger.  One converter could be set to 4.5V for Menards/Lemax/Miller lighting, a 2nd converter could be set to 12V (or so) for the ubiquitous LED strips to light passenger platforms, tunnels, roundhouses, whatever.  And so on.  And it's not just lighting.  Many accessories can operate just fine on DC power even if the manuals only speak of powering via 14-16V AC-Accessory voltage; the manuals were written long ago when DC power was nowhere to be found on O-gauge layouts.

thank you all for your reply`s. great information.

Buck Converters here priced at .37 cents each shipped...no voltage meter but at these prices who cares?

Great price, but you will have to add additional bulk capacitance on the input side after the bridge rectifier for any significant current draw.  The 100uf won't be nearly enough for more than 100-150ma input current.