Place the train for the shot and position the view, then press the PrtScr key. Trainz will create a .tga or .jpg file (depends on which version of Trainz you're using) in the Trainz Screenshots folder. Screenshots will be named/numbered consecutively so you can take as many as you like.
You can also get a program called Fraps, its free. It will take pictures, make movies and tell you the FPS "frames per sec". That's what most of us use.
