I'd avoid Dell or any other OEM (pre-built) unless you are certain you can't build one yourself or at least have one built for you. For a grand, you can build a decent, i5-based machine with SSD and a good graphics card. If you have a retail copy of Windows, so much the better; if not, you can buy one and use it for this machine and your next, rather than essentially having to throw your old copy out with your old machine.
Here's what I would buy:
i5-2500
Gigabyte GA-Z68AP-D3 Motherboard
nVidia 560Ti
Kingston 4Gb memory
Kingston SSDNow 128Gb SSD
Corsair 850W power supply
Cooler Master HAF-922 Case
Lite-On iHAS-124 DVD burner
That roughly works out to about $950. I haven't factored in the cost the builder would charge or a new copy of Windows as I don't know if you need that or not. If you do get one, get Windows 7 64-bit and DO NOT buy an "OEM" copy; buy a "retail" copy, preferably of Windows 7 Professional 64-bit. You can save $100 by getting a cheaper video card, such as a 550Ti. You can also save $50 on the motherboard by getting a lower-end Gigabyte board, the GA-H61M, if you need to.
If 128Gb of storage isn't enough for you, tack on about $80 for a 500Gb, 7200RPM Sata-II/III hard drive. 128 should be enough for Windows, Trainz, and maybe a few other programs.
I do recommend, if you really want to get the most out of your Trainz machine, relegate the old machine to internet and email duty so you don't have to install firewalls and anti-virus on the gaming machine, since that can greatly affect performance. You SHOULD be able to reformat the old lappie (if you didn't get recovery disks, you may be able to still buy them from the manufacturer or on eBay). You don't need a lot of horsepower to browse the internet; you do for games.
Edit: You can probably safely go as low as a 700-Watt power supply, especially if you get one from a quality manufacturer such as Corsair, Seasonic, or Antec, and save a few bucks, though I recall seeing an 850W Corsair on sale recently at Tigerdirect.