Of course, .th2 can hold many scraps. Scrap projection does not matter. Simply open your .th2 file and insert the new background image. You can place the image by dragging (double right click + drag) or, more precisely, giving coordinates in pixels for upper left corner of inserted image and pressing “Move to” button.
Draw your lines, reference your stations, etc. As you can see, all this work is exactly the same you did for plan.