My first railway sim was MSTS which I picked up in a charity shop, along with 4 or 5 of the add on packs, for a fiver. I played it for a while, before concluding that it should have been called MS Train Driver, as there didn't seem much that could easily be done.
My next was the other sim (which name shall not be mentioned here). I found out during the install that I needed a Steam account to run it. Well, Steam is free, so I thought "Why not?". \same thing - just a driver.
Then I saw Trainz 2006 in a pre-owned shop we have here in the UK, called the Computer Exchange, for £3, so I got it home. Have you ever walked through a busy city centre on a Saturday afternoon carrying a bright red bag, with the initials "CEX" written on it in bright white and very large letters? I got some strange looks. That's when I got hooked.
Trainz 2009 was acquired the same way, but this time I had my own bag. And this became my favoured addictive drug, and still is in many ways.
Then, while in Steam one day, I saw Trainz2010 and 3 DLC packs on sale, so I got them. Realised soon enough that there would be patching problems (through reading the forums), so TS12, and another copy of 2010, came direct from N3V and I've never looked back.