Well, looking at the Steam's Store Page for RailWorks3,
http://store.steampowered.com/app/24010/ ,it now comes with:
Cutting edge graphics and detail
8 Realistic lines
16 highly-detailed locomotives including Hitachi Super Express
More than 70 scenarios
Realistic weather and seasons
World Editor to modify and build routes
Career System leader board
Driving tutorials
And for a few dollars more, you can get the Deluxe version, which includes the "Horseshoe Curve Expansion Pack" that comes with "EMD GP7 and F7 locomotives plus unlock-able scenarios which allow you to work your way up as an engineer on the Pennsylvania Railroad."
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So, there's already lots included and to do with the base RailWorks3 program and one could potentially get hundreds of hours of enjoyment just exploring the routes, scenarios and trainsets that come with it. So, just start with the base program at first should be fine.
As for DLCs / Addons, it's all completely optional, and a User might only decide to get what catches their interests. And here's a tip; A lot of us more patient and budget-minded RailWorks Users tend to wait buy any of the Official DLCs that we might have our eye on, during the occasional Sales that Steam tends to have anyways. (For example; not too long ago, during the Autumn Sale on Steam, RailWorks3 and all DLC was 50% off, and had one day it was all at 75% off. And more recently, there was a bonus 50% Off coupon that RailWorks Users could use to buy a choice of DLC).
Speaking of which Steam tends to have a good Sale right around Christmas (chances are the Christmas Sales on Steam will start very soon), and though I can't say for sure, chances are that RailWorks, and the various DLCs for it, will be on sale then for a good discounted price.
Also, if you look around, there's also lots of Freeware Addons available too; For example, the RailWorksAmerica File Library has a nice selection of freebies.
There's also some independent Developers, like myself, that offer some free addons as well (My own site here:
http://www.rryard.com/ , and though I do have a couple of PayWare Addons listed there in the
Routes and Scenarios section, I also give away several Scenario Addons for free as well).
Cheers!
