These are not 'sim' games; they are old-fashioned low-tech railroad-themed games. Fun for passing the time.
http://www.arcadetown.com/holidayexpress/gameonline.asp
http://kurohako.com/games/railz/
for younger ones: http://pbskids.org/cyberchase/games/decimals/decimals.html
http://www.unoriginal.co.uk/railroad.html
http://www.terragame.com/downloadable/railroad/eep_virtual_railroad_pro_3/index.html
http://boardgames.about.com/od/toppicks/tp/train-games.htm
Now you all have your Christmas presents.
Here's a simulator one I like: BVE. It's a cabview train simulator with lots of add-ons. It is mostly focussed on rapid transit, but a US freight route I've found works well, too. Though seeming simple at first, it is very realistic, with screech sounds around curves and sometimes during braking, clickity-clack and cur-chunk sounds over switches & junctions, dispatcher announcements, and much more.
The cons: it's just a straight-out-of-the-cab simulator, no other views. And, the other trains you see out the windows are not really moving (although sounds sometimes tell you otherwise...).
But for what it is, it is surpisingly realistic. And it's free. You can download it here: http://mackoy.cool.ne.jp/ (the official site) There are many other routes available, and you'll be amazed at how fun it is. I sure was!
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