Well maybe I can help. To run most any modern game effectively your are going to need a strong video card under the hood. I use the Geforce 6800 but you could probably still enjoy Trainz using a lower end graphics card like a Radeon 9600 which comes for less than $100 now days.
As for the system RAM, I would suggest having at least a gig. A gig and a half would help if you can manage it. For the processor and motherboard look for something that will support and an Accelerated Graphic Port or better yet a Scalable Link Interface which will allow you to make two graphics cards act as one when required. For the processor I would recommend an AMD 64 but I'm sure Intel has a comparable chip set.
For parts go to www.tigerdirect.com
Here is my computer info.
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System Information
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Time of this report: 8/15/2006, 14:23:21
Machine name: INFINITY
Operating System: Windows XP Professional (5.1, Build 2600) Service Pack 2 (2600.xpsp_sp2_gdr.050301-1519)
Language: English (Regional Setting: English)
System Manufacturer: System manufacturer
System Model: System Product Name
BIOS: Phoenix - AwardBIOS v6.00PG
Processor: AMD Athlon(tm) 64 Processor 3200+, MMX, 3DNow, ~2.0GHz
Memory: 1024MB RAM
Page File: 385MB used, 2074MB available
Windows Dir: C:WINDOWS
DirectX Version: DirectX 9.0c (4.09.0000.0904)
DxDiag Version: 5.03.2600.2180 32bit Unicode
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Display Devices
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Card name: NVIDIA GeForce 6800 XT
Manufacturer: NVIDIA
Chip type: GeForce 6800 XT
Display Memory: 256.0 MB
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Sound Devices
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Description: SB X-Fi Audio [A000]
Min/Max Sample Rate: 1, 192000
Static/Strm HW Mix Bufs: 128, 128
Static/Strm HW 3D Bufs: 128, 128
HW Memory: 0
Voice Management: Yes
EAX(tm) 2.0 Listen/Src: Yes, Yes
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Sound Capture Devices
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Description: SB X-Fi Audio [A000]
Default Sound Capture: Yes
Default Voice Capture: Yes
Driver Name: ctaud2k.sys
Driver Version: 5.12.0001.1144 (English)
Driver Attributes: Final Retail
Date and Size: 10/29/2005 19:16:24, 439680 bytes
Cap Flags: 0x41
Format Flags: 0xFFF
You can get the same information on your machine by going to Start< Run and typing dxdiag.