Also watch out for the new MS Train Sim, using the same engine as FSX.
If you are in UK(or just interested in British rail like me), check out uktransim.com, they
have tons of stuff for MSTS but also some for Trainz(there are better dedicated Trainz sites though).
It's quite simple. If you are into model railroading, then TRSXX might be the most fun but if you like
a realistic sim then MSTS is the better one. I like both but then I love trains

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The cool thing about Trainz is that it makes designing your own tracks very very easy, there are also thousands of free objects etc you can use. Mainly US, EU and AU stuff but there are some cool Japanese stuff as well. Not as good as the MSTS Jap items.
Also. If I could recommend onge single download for TRS. It would b Razorback Railways, it's free and totally amazing. It's a huge very realistic Australian map with hundreds of activities and awesome terrain. It is simply a must have for any TRS owner.
There are those who can't understand the novelty of trains but then they never grew up with model railorading

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Actually. Auran had this long term goal to do some kind of economic sim out of Trainz but I am not sure it would work, the editor would have to be far easier to use.
Deus Vult.