BIG PICTURE: Most people with a desire to know about trains want to know:
-What kind of trains am I seeing near me, or what trains have I seen in my life?
-How do locomotives work?
-History of railroad lines & systems
-How to identify types of trains? (locomotives & rolling stock)
Youtube is probably the most fun way to learn about trains, big exciting videos with (hopefully) lush visuals to kindle interest at any age and comprehension level. Here's some of my favorite channels/videos to help you learn efficiently and potentially grow what'll definitely be a lifelong passion.