These are the specs for TS 2004 taken from an old Auran webpage
Windows 98/2000/ME/XP
PIII800Mhz • 256Mb RAM • 32Mb video card (Geforce equiv.)
Direct X 9 or higher • Direct X compatible sound card
8X CD-ROM • 56K Modem for Internet access
They are pretty low requirements so TS 2004 should run on a modern machine. Later versions of trainz really require a dedicated graphics card. Check you have up to date drivers for your graphics system. Also post details of any errors you are getting and we might be able to help more.
Edit: Please also post your computer specs and OS version.