I have Railworks, which has recently been updated by Steam and is now called Train Simulator 2013. I indeed created a Steam account and purchased and downloaded it from there. There is a basic standard version and also various premium packaged editions which include some extra scenarios and stock at a higher price.
The initial download is huge, several, several gigabytes, and takes forever on a slower connection. Mine took a day and a night on my old 384kb/sec line. Steam, when installed, runs automatically each time at computer startup (it could be disabled if you knew how). It logs into your account and automatically downloads updates for both itself and your installed games without so much as a question. RW updates are large (perhaps not as large as the initial install) and chew up bandwidth so if you are on cellphone data (3G, HSDPA or not) be prepared for a suprise huge bill. If you are on a capped ADSL line, be prepared to hit your cap without realising it and have to buy a top up! Steam is designed around first world countries with high bandwidth and unlimited internet access. Of course whilst the updates are in download, you find yourself wondering why your internet browsing and other downloads are so slooooow. The upside is that your simulator is always up to date and, bandwidth charges excepted, the updates are free.
You need to launch RW from the Steam app. The Steam app itself can be run up whilst not connected to internet and contrary to some information, RW can then be run without difficulty. Being on-line does confer a few benefits, such as the updates, a multi-player environment (I have never used that facility for RW and have no idea how well or if it works) and Steam also keeps track of how often and for how long you have run your RW.
RW itself I think is pretty good, albeit that some of the dynamic modelling is simplistic and poor (for example, signalling on an 80 mph line placed incorrectly and so close together that even if you hit the brakes immediately and hard as you approach a signal at caution you will never stop at the next danger signal). You may well get bored with the standard scenarions and want to add new routes and stock. Beware though that, like MS Flight Sim X, such add-ons from commercial developers (which generally seem to be the most complete and best -- they have to be to sell them) can each cost more than RW itself, making this an expensive hobby if you want variety.
BTW you also need a good fast PC to run RW. It is intensive and a graphics accelerator card is essential.
I personally prefer MS Flight Sim X essentially because there is a large amount of real, real-world stuff available (like proper charts, "airac" real-world navigation updates and real-time weather) which costs almost nothing and seriously enhances the experience taking it away from a game towards a realistic simulator. FS is much more realistic and not as limiting as a Train Sim. Commercially available add-on aircraft are reasonbly priced if you consider the precise and massive detail that has gone into them, right down to working panesl, instruments, flight computers, training manuals, checklists, etc. If you do enjoy FS, then I do recommend you join VATSIM
http://www.vatsim.net (free) whose members aspire to create as realistic as possible multi-player scenarios including manned air traffic control at major airports and ATC areas around the world and implement proper flight planning and piloting procedures. The local SA VATSIM region, VATSAF
http://http://www.vatsaf.com/, regularly organises manned local ATC and fly-ins for those who have become skilled enough and care to join in (not only from SA). FS has a much bigger following and has a much better, broader and more mature eco-system around it than RW.
The reality is that both RW (ITS 2013) and FSX, taken to the max, take up lots of time, block other more productive leisure persuits and do not earn any real money, so I personally have to stay away from them as much as I can.