There are several O gauge track systems. If you do a search here on the forum you should find several topics, including some folks actually trying to do noise studies. Lionel fast-track is actually one of the loudest and most expensive. Fasttrack is great for floor layouts and for kids to take apart and reassemble over and over, but its hollow base ad square hollow rails are really loud. The quietest is probably atlas solid 3 rail track with brass rails, although also very expensive. For a good compromise use Gargraves - Hollow rails but wooden ties, low cost. Install foam or cork road bed under the track. If you can then left the track float onto top of the foam/cork. I.e. glue down the roadbed, then glue the track to the top of the road bed. Use as few screws as you can, mostly in the corners. Every screw transmits vibration from the train to the solid surface, which then acts as a speaker.
If noise is a huge concern, as it should be honestly, then consider a decorative noise abatement treat to the underside of the shelf. You will also find that some cars are louder than others to operate, depending on wheel type, weight, how solid the construction is etc.
the biggest thing is to always have fun - let the various challenges be neat little learning exercises, not chores. make sure at the end of every day you can run a train so you have something to smile at 
