Trainz will run on a variety of hardware. Middleton for laptops for example will run on Intel integrated graphics series 4000 or later.
No hardware exists that will run any content with the sliders set on max. Some user created content is extremely inefficient.
So then you get to what is reasonable.
The GPU is the most important,
https://www.videocardbenchmark.net/gpu_list.php ideally you want a 3d Score of 10,000 or more.
N3V uses some features of nVidia so I'd stay with nVidia.
I run an RTX 2070 which has a 3D score of 16,000 and I'm fairly happy with it. These days I might look at an RTX 4060 but look at the cost performance side as well. To get an increase of 10% in the frame rate the cost starts to more than double as you move into RTX 4090 etc.
SSDs don't affect the frame rate very much but the loading time is a lot faster so drop one in and run trainz from it, nvme is fashionable at the moment.
Cooling is important as is the power supply.
Personally I'd lean towards a refurbished Dell workstation 16 gig or more with a w xeon processor that can be upgraded to win 11. the e processors can't be upgraded, then shove in a decent graphics card but new Dell machines aren't too bad at least their cooling is worked out.
CPU it's a matter of balance, a lower end GPU would be happy with an i5, an RTX 4090 needs a higher end CPU to keep it fed.
Cheerio John