Until GreyWolf finds his tutorial, there were a couple in last month's Trainz Community News. One was about creating a brick texture, but just replace the bricks with anything you like. You can use either Paintshop, Photoshop or IrfanView.
Make sure you read about Perspective Transformations tutorial, you're going to need to do this sometime.
This month's TCN which should be out in a few days (running late) has a very interesting tutorial on making wall, floor or pavement textures from a single brick, lump of wood or concrete. Also well worth a read when it comes out.
http://issuu.com/trainzcommunity
Note: The Issuu site has a strange way of displaying big projects. The white dots under the magazine represent every 20 pages. When you start, only the first 20 pages are shown. Click the second dot to get the next 20 etc.