What size wire do I need?

 I still haven't  read enough about track power so far. Keep in mind my lack of understanding for anything electrical.  It still makes me wonder what size (gauge) of wire I need to be able to run my trains outdoors. I have heard that 14 gauge is OK. Is this true? Does the power supply dictate what gauge wire I should use? What about number of locomotives running at the same time? What will the maximum length of track  that 14 gauge is good for?  I'm sure that the longer distance I go, the heavier the wire should be.  Is speaker wire a consideration, since it is two wires side by side already? Should I use separate wires instead? Is stranded wire better than solid to solder to the track? Do I have to bury it in conduit if it is not rated as underground cable? Would it be OK to just bury it in a few inches of sand to prevent any cuts to the insulation? How deep does it need to be buried? Just under the surface, or is it better to be like 6 inches down or so? Does the frost heave  break any wires? How many feet apart should I feed the track? I would guess that I should solder all joints and connections also.  What should I use to waterproof the connections underground? Will liquid electrical tape work, or is there something better?
  OK, I'm sure I have exceeded the amount of questions for one topic! I look forward to all your ideas and thoughts. I plan to lay track this spring, so I need some ideas before I start digging and live to regret what I do ! Thanks.
   Kris