The GWML, the route represented in the Great Western Express DLC, is one of the busiest routes in the UK, especially around important stations on the line included in the DLC, but sadly they're rather lifeless in TSW. I'm going to be using Reading to illustrate this. This screenshot was taken at 15:30 (in-game time), and as you can see here, the station concourse at Reading, a major hub for both local and long distance services across London, the Home Counties and into the Westcountry, is deathly quiet. Whereas in real life, the covered section above would be populated with people, with plenty of people walking around on the platforms as well. Meanwhile, this branch of WHSmith, a favoured shop for newspapers, books and snacks, seems to repel everyone who goes near! You can almost hear the tumbleweeds rolling around! Of course, it would be taxing on lower-end hardware to represent these stations as they are in real life, but a buff to the amount of pedestrians spawning in some of the busiest stations at least would help a lot with the realism, plus it would give a lot of life to the architecturally fantastic buildings you can walk around in the GWE DLC. It's also notable that despite the lack of NPCs walking around the station, they're capable of interacting with it a lot more than you can. Rarely you'll find them sitting on a bench instead of standing, which is something the player can't do. In my opinion, it would be really cool if you could do stuff like operate turnstiles, sit on benches and chairs, and maybe even carry out some tasks such as drinking a cup of coffee or opening doors to shelters when the weather's bad.
Imo a quick improvement to add to stations would be an actual station background audio. I think this would help a bit, for instance at paddington, where you don't know if you entered a cathedral or an actual station that sees 35M people per year .
A traffic system (wouldnt even need to animate for London as traffic rarely moves, same for LIRR) a loco dlc that adds a new dimension to services like RSR, MSB and now NTP. Lots and lots of people on platforms.