You can think of it as being the same to NARS as BROS is to UKRS. The goal here is realism and eyecandy.
Here's a list of hopefully-planned features - some may require additions to the NewGRF spec and might take a (potentially infinite) amount of time. Some will require a bunch of artwork:
- Realistic multiheading - reduced crew costs for booster units. Additional units that don't have an MU connector will be unpowered.
- Realistic B-units - purchased separately, reduced speed if leading a train.
- Slugs and related items - Available in cabbed and cabless varieties. Cannot move independently. Cabless varieties reduce speed if leading trains. Slugs increase the TE of compatible adjacent locomotives. Some varieties work only at low speeds, others continuously.
- Eyecandy non-revenue rolling stock - MoW vehicles, special trains, etc. Entirely optional, of course.
- Realistic coupler enforcement. Your fancy high-speed rolling stock can't be hitched to a road locomotive without an adapter car in between. For instance, an Acela Express locomotive has an AAR coupler up front (it's there - hidden under a panel), so you can tow it - but behind it, you can only have compatible railcars.
- Explicit livery refitting for all locomotives and appropriate rolling stock. (Uses the NARS regearing cargo, but does not break it)
- Early start date (think horse-drawn)
- Experimental locomotives and trainsets have very short availability windows and are massively expensive if not purchased during the preview period. (Ex. The Bombardier JetTrain)
- Multiple classes of passenger service - first class coaches are smaller but pay higher rates and so forth. Refrigeration technology will improve over time using the same mechanism (Needs recent trunk).
- The Bombardier/Acela Express family (Acela Express, Acela HHP-8, JetTrain) in both liveries. The HHP8 can haul both Acela coaches and regular stock (this isn't strictly realistic, it's just that way for demonstration purposes)
- The EMD F7 family, in all the liveries that were available in the original US Set.
- Boxcars and coilcars, just to give the F-units something to haul around.
If you did graphics for the original set, check the credits on the NewGRF description and make sure you're there. I think I got everyone.