Ho Scale Trains
HO scale trains are the most popular as they allow for a lot of building and track work as it's not too big for the standard home, but it also offers a great deal of realistic detail as well. You can create elaborate track work for HO model trains and plenty of curves as well which you can't do with other scale models..
HO scale model trains like I said before offer a great deal of realism in the design of the locomotive itself, it also has a wide variety of train cars and tankers that you can purchase. And there are plenty of realistic layouts available as well.
With the on set of modern times, things like digital command and control centers are available to run your trains on. Digital sounds are available to make it sound just like the real thing. HO trains are just like the real thing, just smaller!
It's easy to shop for HO scale model trains and train parts. Every good hobby shop carries a wide variety of things you can purchase for your train set. Of course the Internet is full of places to buy items you cannot find else where. There is an article on this site about buying model trains online, you should really read it.
Model trains and model railroading is a very enjoyable and rewarding hobby. You really feel like you are creating something special when you start to create a model train layout. HO scale model trains are a great place to start with your hobby as well.