Gads, where to start! Do you like steam or diesel, North American or British? There is so much stuff out there, both payware and freeware.
I prefer big American steam and diesel, altho I do have the Tornado and the Scotsman. But the Big Boy, the Challenger, the Berkshires, the SD70 pack, the Dash 9's, the SD40-2's, the GP9's, all terrific!
Routes: Portland Terminal, did you get Horseshoe Curve with the base game? Lots of free stuff here in the file library. Lonewolfdon's route is really good, the one from Doug (Heavy Chevy) is also coming along nicely, don't overlook Toripony's C&O Allegheny.
That should keep you busy for a while until I think of more.

Oh, and

Ryzen 7 2700K, Asus Prime X570P, 32Gb DDR4, 2x 1Tb M.2 SSD's, RTX2060 6Gb, Occulus Rift
Win 10 Pro 64bit, keyboard/ mouse/ wheel/ pedals/ baseball bat
Security Coordinator on the Battleship Iowa