Welcome Steamer to the addictive world of Trainz!
There's an old thread in the Screenshots section that has others' ideas of fictional railways they've made for Trainz, so it might be worth looking at:
https://forums.auran.com/threads/make-your-own-train-company.46290/ (some of the posts the images are no longer showing however).
You don't have to do so, but creating a fictional railway company that is believable and realistic seeming does require not just a certain amount of imagination, some skill in writing can definetely help too, as the same concept behind developing the setting and characters of a story applies here (in writing it's what's known as 'worldbuilding'). It's best to just start with the basics, i.e. : the location, time period and things like "is this going to be a shortline or a major Class 1?", "what sort of locos and rollingstock do I like that I could be able to include in this and still be believable or fits the era?", what does this company move as it's whole reason for existing?"
Once you've answered those questions, you can start to flesh out the ideas more, like many of the ideas in that thread, and come up with histories and present situations for the railway company that will expand on those ideas.
Really you're only limited by your imagination and how far you're willing to use "artistic license" to bend believability (like, having a present day set company that still runs mainline steam fulltime is a bit of a stretch, but a shortline that has one or two steam locomotives on it's books for tourist trains and still occassionally rosters them on a revenue freight is still believable because it is possible in reality). Of course though, you can also have completely off the wall ideas (like an intergrated company that operates the entire US, Canadian and Mexican networks under the one operator), and there is nothing wrong with that either! It's your route, your train company and what it does is up to you!
But if you want it to look realistic it does need to at least try to be somewhat believable.