I'm afraid that on the same Mac computer (late iMac 2013) MacTANE and TANE are two different beasts. MacTANE is barely useable with a stutter every couple of seconds. However, in BootCamp TANE is a useable commodity with shadows and a smooth running experience.
Where to lay the blame? Has to lie at Apple's door I'm afraid, OpenGL is moribund in macOS and Metal is an unknown commodity.
A question for NV3 will Metal be featured in a future mac version of the software, apple haven't even bothered to update the os to take advantage of the latest versions of open Gl, which is a disgrace, they are really moving away to non pc devices and with no update to the mac pro in sight, i feel that they are abandoning power users.
i'm in two minds, i still find TANE a pain due to the lack of assets, i have both the mac and pc versions and i don't use either to create routes ( which is my main interest) ,due to the lack of said assets, for instance, i can;'t find a decent sagebrush, how on earth can you model the arid zones of colorado or new mexico without sagebrush ? )
Open GL is a big handicap, a better graphics card makes virtually no difference to running the game on either of my mac pros , one with the 4gb gtx 970 vs my older one gb 5770 card . i also have a PC, but have endless difficulties getting any version of trainz to run on that too. in this case its installing assets and reliability that is the issue .
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I must admit, i often get quite depressed using the software on either platform and either version i have , it seems pointless to make routes for 2012 , but any routes i can make for TANE will be devoid of so many assets i want to use that i will not be satisfied with the results i create and if i begin to make assets then i will never have any time to create routes, the very strength that makes trainz interesting ( the enthusiasts own creations ) is also one of its greatest weaknesses . an easy conversion program to take assets and make then compatible is sorely needed, but i imagine the complexity of creating such a program would be formidable.
Unless NV3 can overcome this hurdle of important items being unusable for the game,often for years i can see many people being discouraged from using the game on either platform .