Check out MTH trains, http://www.mthtrains.com/ Note that they have several lines of trains. RailKing is the more affordable, traditional and has starter sets. They also have a collector line called Premier. The Premier line s oriented to adult collectors who want accurate models of real trains. (For example, engines whose cooling fans actually turn.)
Here's a decent link to get you started and includes information about other manufacturers: http://henrykautz.org/Trains/getting-started.htm
I have the O and have owned it for over 3 years now and love it. I keep it up all year around and when I want to go on a vacation in my head, I just start the train up and get going. Never had any problems with it. I do not have the Polar Express or the Harry Potter just the Lionel O guage. I do believe it is made quite well and would buy it again. Nothing is the same anymore but this I like. They have replaced so much with plastic for what reason I do not know. Bet most of us wished for the good ole days when things lasted forever.