Wanting to know about trains

I want to learn about trains where do I start? Like I want to know a lot about it but like it’s so confusing what should I learn first. Also I looked up the Mallard trains and it shows “LNER Class A4 4468 Mallard” what do the numbers and letters mean? I see it a lot with trains? That’s what confuses me about yalls posts because it shows a lot of complicated names and I don’t know what it means.