If you are going to make blanket statementa about cmputers, its best to be sure of your ground before you do so, you are perpetuating a perception that isnt entirely accurate.
Anyway, I just get fed up with people making generalizations about mac performance,my point is , for far less than the price of a new iMac you can get a unit that is far more upgradable and easy to work on , it might be better to find that out, rather than keep on repeating info that isn't accurate .
I've been down the road you have taken , had an imac, found it sorely lacking in the graphics department and moved on to a better machine which will run Trainz very well on windows too.
This is the thing that most people continue to ignore, that you can run bootcamp and have good gaming machine as long as you ensure you a decent card installed when you buy, because almost of apples other computers wont let you upgrade after purchase, one reason I've stopped buying new macs as they are quite frankly - a ripoff if you want a reasonable os x gaming experience unless you ensure they have the 8gb cards. To buy a top spec'd 27 inch imac here in Australia would cost me nearly $4000, I've bought a better , albeit older computer for a third of that price and even if i had to replace some of the more expensive parts, i would still be ahead , my point is, if you want to have a decent mac experience using Trainz, and don't want to spend a fortune, go for the old mac pro- 2010 or 2012 versions , so much better then the iMac, apart from the weight