Hey carrwoodrr,
To take a screenshot, press the "Print Screen" key at the top right of your keyboard. Once done, open the directory where your Trainz installation is. (Like C:\Program Files\Auran\TRS2006\)
Now find the "Screenshots" folder within. Inside should be all the screenshots you've taken. You'll need a program like IrfanView (which is freely downloadable) to convert the .tga files to jpeg. Then you can upload them to a place like photobucket or Imageshack.
Cheers,
John