Open Content Manager, add a filter for category - Texture Environmental. Add a column for build version if you don't already have one, and sort by build version.
TS2009-TS2012 = builds 2.4 through 2.5.4
Clone an existing ground texture (from the DLS), use a build number relating to the version of Trainz you're using, I assume it's TS12, and open it for edit in content manager. There will be one image. Open that image in an image editor. Preferably GIMP or Photoshop, as you will likely need to learn how to make an image seamless (tileable), and/or bring in a pre-existing seamless image from the internet, or other source.
If the image is in .TEXTURE format, then you will need to use PEVTools Images2TGA to convert your new image. Found here >
PevSoft
Try to find a fairly high resolution image, 1024x1024 or 512x512. The better the resolution the less blurry it will look at larger scaling sizes. When editing, all images need saved in power of 2 sizing, so, 64x64, 128x128, 256x256, 512x512,1024x1024 and so on. Once you've finished editing your new image, replace the image in your cloned file, rename it whatever you like, i.e desert_rose.tga, grass1.tga etc
In the new asset folder, open the config file in Notepad, change the existing texture name to your new one, and change the username as well, give it a fitting name with maybe a two or three letter initial, i.e SAR-Desert Rose 1. This will make it easy to find when you're in Surveyor.
Change the author name to yours, then save/close
Commit the new texture by right clicking it in Content Manager, choose submit edits. go into surveyor, place your new texture, see how it looks, and if necessary open it in Content Manager and edit the image in your image editor, changing color range, brightness/contrast or whatever the image needs.
There are tuts for using GIMP and photoshop all over the internet. You'll need to learn how to use layers, make tileable images, change brightness, contrast, edit color saturation, and possibly adjusting color balance, vibrancy and other edits. Take your time
Have fun