A wikipedia search for "rail car" brings you information on everything from RDCs and Doodlebugs to control cars, stock cars and container well cars.
http://en.wikipedia.org/w/index.php?title=Special%3ASearch&search=Rail+Car&fulltext=Search
Here's info on reefers, boxcars, tankers, stock cars, hoppers, autoracks and more.
http://en.wikipedia.org/wiki/Railroad_car
And if you're an uber railfan, check out some old school pics of trains from the 19th century here:
http://www.old-picture.com/trains-index-001.htm
For anything else, the Google is your friend.