Help/Paint Tab
From TrainzOnline
Click on the Paint Tab (F2) to open the Paint Menu. You will see a number of default textures, grouped by region. Use the slider button to scroll through the list. Click on one of the textures or write its name on the text box, and you will see a copy of it appear in the small current texture window. Click LMB on the terrain and use your mouse as a paintbrush to paint on the texture.
