Welcome to TRAINZ! Amazon has Alienware and some other great gaming computers for less than 2K. I would go for an I9 Quadcore, and RTX 2070 Graphics as mentioned by MP242 above. In fact, I would go with the rest of what he has above. And everything comes with Windows 10 now (Windows 7 is out of support), and it does well if not overly bloated down. I have an Alienware I9 Quadcore with a GTX 1080 graphics card, and it makes a great heater when TRAINZ kicks the cooling fan up on high, which it does with great regularity. From reading the forums the last couple of years, I have come to the conclusion that TRAINZ is somewhat of a computer killer, so don't go cheap. There should be some other folks that can recommend the AMD processors instead of Intel, but I don't know how they compare.
If you pick up Trainz Model Railroad 2017, I bet you might find enough assets to rebuild your HO layout in TRAINZ, and there are plenty of folks here to help you!