The reason I love model trains, and O in particular, is that I don't need to be good at it. I can just enjoy it and keep adding to it.
Why I model a particular theme, time, name? I like to run trains that are from the first half of the 20th century. I ike to focus on NE names since that is where I live. Modern rail has no... romance about it compared to the early 20th century.
I think the 1930s-1950s was the golden age of rail with both diesel and steam running concurrently; the boom and progress during the post war years spurred the increased movement of people (thus also purring the increase in air traffic leading to apassenger rail's demise). The focus on the early and mid 20th century is where everything that is great about rail came to a head.