A couple of things to remember, SATA drives are limited by the interface to around 450 to 550 MB/s, the Seagate drive is 275MB/s, sustained NVME on PCIe 4 can hit 14,000 MB/s however you need a motherboard with PCIe 4 to get there. Gen 3 SSDs at 4,000-5,000 MB/s are fast enough for PCIe 3 motherboards.
Second hard disks have two things to watch out for, rotational ie slurp up everything on this track and head movement to another track which means a delay if you aren't on the right track to start with. For big chunks of data such as TS22.exe then the head just sits on the track and slurps it up. For lots of small files the heads have to move back and forth from track to track which is an overhead. SSDs don't have that problem. So in the ideal world it makes sense to put the big contiguous files such as ts22.exe on the hard drive and the data folder on the SSD.
The cache doesn't really help much on a hard drive except when writing, and in Trainz you're really doing lots of reads. If you're lucky the file will be in the cache but with lots of small files scattered across the drive you'll need to access the physical drive.
Oh and finally big SSDs make more sense as the wear is spread over a larger area so a brand name 8 TB SSD would be nice.
Have fun
Cheerio John
