Thanks for replying to many of my questions, SteamGene!  I don't know how to find the MR rating of pulling capacity for a locomotive I am considering.  Some descriptions say all-wheel drive & others don't say anything about the drive.  I will check the websites you recommend.  As far as pulling 20 cars, I just want some reserve power.  I probably would stop at about 15 for freight & 10 for a passenger train.
  I read that Walthers has adapters to connect their PowerLoc with other track.  Now, I'm worried that different brands of locomotives & cars might not have compatible couplers.  I hope the Bachmann ore cars I ordered will couple to the Walthers set I have.  I have not seen any basic information about what to consider when buying locomotives, cars, track, & transformers for a compatible system that runs well.

  RAM, my local library has just two books on model trains.  One is 29 years old.  Neither answers the questions I posted here.

  Janecker2, I will get a copy of Walthers Catalog.  I searched Amazon.com for books on model trains, was able to see the table of contents for several & do not think they would address many of my questions.

 I still have two Marx train sets my parents bought me in the early '50s.  I guess they are called "tinplate" & seem to be about O size.  I decided to try scale model trains & chose HO, because it is almost 50% smaller than my old trains & fits better into a room.  We have a finished basement with a low-pile carpet.  I set up my old trains in the largest room, but I will remove them as my HO layout grows.  I have space for about 6'x13' of track.  At a local department store, I found a Stock Car Express HO train set by Walthers.  It has a passenger style diesel locomotive, 5 boxcars, a caboose, & 12' of power-loc track.  I bought it for $46; Walthers' website prices it at $90.  The only thing I don't like is that they have NASCAR logos, instead of genuine railroad logos.  Eventually, I want to get a freight locomotive & get passenger cars for the locomotive that I have.
   
   The nearest hobby shop is a 50+ mile long round trip, so I can't drop in whenever I have questions & want to compare products.   So far, I haven't found much information for beginners that answers my basic questions.

   I just ordered 6' of Ezlock track & then wondered whether or not it is compatible with my power-loc.  I like the track that comes with its own roadbed, but it is expensive.  Is Flex track cheaper?  Does it require making a roadbed?

   Do you put freight in your freight cars?  I want to do that.  Because I am originally from Minnesota's Mesabi Iron Range, I want a Great Northern locomotive.  I ordered 3 D, M, & IR ore cars & hope to have them carry something to simulate iron ore.

   In Model Railroader (July, 2008)I read about a new locomotive that can pull 100 cars.  I would like a freight locomotive that can pull about 20 cars at a good speed & with several ounces of freight.  I am considering the Great Northern SkyATH-94159-2, but sellers' descriptions often don't tell me what I want to know, like how much pulling power it has.  My present locomotive does not have all-wheel drive, which I guess delivers more power.
  If I buy another locomotive, do I need a separate transformer for it?  How do I know what transformer to get?  I'm guessing that a more powerful locomotive needs a more powerful transformer than the one that runs my present locomotive & 6 cars.
   
   If one can't run two trains on the same track, can the two tracks intersect or must one go under or over the other?