I got into modelling by joining a club.
When I joined, I decided to model the Chessie-Seaboard merger era since my first locomotive was an Atlas ATSF S-2 and my first Athearn loco was a Chessie System SD50. Early CSX was also appealing as that is the railroad that runs by my house (as well as the club)
However, the club's president owns enough stuff to fill at least 3 hobby shops and he models CSX. He pretty much owns more engines than CSX does in the present day. Instead of competing with him for addresses, identical locomotive numbers, etc, I decided to model Guilford Rail System (now Pan Am Railways) which is also a local road. Since Pan Am (up until recently) ran nothing but clunker EMDs (GP7s, GP9s, GP35s, GP40s, GP40-2LWs, SD26s, and an SD39) (now, anything that is not a GP40 series engine has been put in storage and they have recently bought a bunch of SD40-2s from KCS)
So, I decided to expand into NS as Pan Am had gone into a deal where Pan Am would haul NS's trains with NS run-through power. After that, I just took off modeling both NS proper and Pan Am Railways.
At the club, my friend (known as ST214 on this forum) models Conrail, another member models the local shortlines in addition to BNSF, another member models UP, another member models the steam era of the Boston and Maine railroad, and another member models the Providence and Worcester Railroad.