http://www.train-fever.net/ (Biggest and most active one I know of.)
http://transport-games.ru/index.php/files/category/40-transport/ (All Russian language site but you can use a page translator to get around and they have some vehicles for other countries as well.)
Sadly these are the only 2 big ones I know of but I’m sure with a little searching you can find more. ^_^