To get the kind of rendering you want in real-time, you would need more than the fastest Intel i9-12600KS or the fastest AMD processor available. The video card needs to be something like an NVidia Quadro or RTX graphics card.
Graphics Cards for Professional Desktop Workstations | NVIDIA. These are not cheap either and start about $4k
To break this down in hardware:
CPU = multiple XEON or AMD equivalent processors. Cost, many $1,000 each.
GPU = NVidia Quadro or RTX professional card, Cost, many, many $1,000.
RAM = 128GB or more DDR 5 costing many, many $1,000 or more.
Hard drives superfast, not consumer SSDs, costing many, many, $1,000 each. And you'll need to set these up with a fast PCIe controller.
The motherboard to support these systems is in the upper echelon as well and will cost many, many $1,000.
When done, you think your $350 video card, which really isn't worth that much, is expensive, you're talking here about a system that'll cost probably about $10,000 just to start.
Getting back to Trainz.
Your bottleneck is really your video card in the version you are using. The modern Trainz versions, such as TANE utilize the GPU substantially as the program renders everything and does the calculations. The CPU is used for other tasks such as accessing the content and feeding the GPU, although with a fast buss and lots of memory, Direct Memory Access (DMA) puts the CPU off to the side more and more as the functions are now done between the GPU and RAM rather than involving the CPU as much. TS12 and below was mostly CPU. Your processor did the bulk of the work fetching data and pushing that through the system to the video card, so yeah, you'll see stutters now due to what is called bottlenecking but the slowest critter in the bunch outside your storage, is the CPU with TS12.
The other issue you are facing is the assets. Some assets are made better than others with some that go way beyond what is necessary to render the asset. Why do some users build assets with 100K polygons and use textures that are well beyond what's needed, is beyond our comprehension. TANE will actually complain and reject the asset, but this wasn't always the case. TS12 would load the asset anyway and come to a complete halt then pick up rendering and carry on as if nothing was amiss. In the meantime, we felt the hard stop, stutters, got dizzy from the headache it caused, then we continue rendering.
There's a lot more to this, but in general for what you want, to do it's well beyond everyone's budget unless you're a billionaire.