Trainz runs fine, cost has nothing to do with helping you out, since you have the equipment and game. Trainz has been optimized for Metal and ASi Macs since Trainz 2019 SP3. Trainz 22 does better yet.
Several things to try, make sure you have the latest version of Trainz. After starting Trainz, click on Trainz settings, click on MyTrainz tab, make sure your user name is correct. If you click on your user name, it will tell you if your password is correct. Next click on Install tab, this lists where your user directory is. If your user directory is on a old style HDD like I do, instead of a SSD, performance will not be as good. This is where you can change the directory, after you copy or move the current directory. I recommend copying the directory as to create a backup, and it prevents Trainz from thinking you have a new Trainz install and having to set things up again. More importantly on this tab, is to click on Change Trainz Update Stream. I recommend keeping it on TRS22 for most people. Next click on the Performance tab. This is where the greatest performance changes will be done. This also depends on what Mac you have and what route you are trying to run. The problem is they don't give Mac examples when trying to setup the game to begin with, so we are left to guess what performance level to choose. On my M1 Mac mini, I used to run with Ultra settings, but with more demanding routes I knocked it down to high settings. Better Macs and running off SSD will allow higher settings.
My settings:
[FONT="]Shadow Quality: high[/FONT]
[FONT="]Main Shadow Resolution: 2048[/FONT]
[FONT="]Shader quality: standard[/FONT]
[FONT="]Texture detail: normal[/FONT]
[FONT="]Post processing: high[/FONT]
[FONT="]Water quality: high[/FONT]
[FONT="]Detail scenery: clutter only[/FONT]
[FONT="]Antialiasing: 4x[/FONT]
[FONT="]Use PhysX simulation: unchecked[/FONT]
[FONT="]process objects behind camera: checked[/FONT]