Andy,
It is and isn't important to know what you are running. On one extreme, you can go totally freelance and do whatever you like - Middle Earth Central meets Moon-monorail rock processing plant

.
But most modellers try to fashion something based at least in part on what really existed/exists today. Once that is finalized, or at least narrowed down, you can also decide era (timeframe) and location. This will help you decide on what industries you might want to include. Some modellers do this the other way - what industries are you interested in, and what railways interact with them? In any case, it does help most people to be somewhat focussed.
Try looking at the "Givens & Druthers" link in my signature. This is a series of questions that John Armstrong (prominent modeller some years ago

) used to help develop model railroads he was planning. The additional questions we have posed below in that thread also help you to think about your approach to the hobby - such as are you a serious detail modeller, or do you just like to run trains? Are you happy with minimal scenery, or do you want mountains floor to ceiling.
Give it a read - I hope it will help. If you think it is too serious, then go back to my first paragraph...!
Andrew
PS - In my case, I decided to model the 1920s and 1930s (I like steamers much more than diesels). I chose southern Ontario (my place of origin). I am building my layout in modules, so that I can operate with the local modular group. I enjoy the model building part as much as running trains. I also enjoy the research/history side of the hobby as much as the models (another reason to choose 1920s and 30s).