Here is the front and back of an old flash card, part of an index card deck I created when I was in my Amtrak conductor training class in Wilmington, DE.
This particular card was for a NS Conrail signal. I would draw the signal on the front side and the signal name and rule number on the back.
You just needed red, yellow, green, and black Fine Point Sharpies to create them along with blank 3 x 5 index cards. These flash cards are what inspired this website LOL.