Improving the train system would increase ridership.
1. Make them run every half hour at the top of the hour (counterclockwise) and bottom of the hour (clockwise).
2. Add a station between Hub/Junktown/Necropolis, at Broken Hills, Modoc, Klamath, Redding, and Gordon's Gas Station. (IRL, the tracks already exist, the NCR would just be repairing them, albeit probably quite a bit.)
3. Link the ends together for a complete circuit.
I know the trains are an NICE thing for their own business, but this makes sense because the NOR would have a permanent presence in all regions (kinda like building a military base in enemy territory), and for trade purposes directly. New Reno would still be off-limits as would Enclave, but the other points of interest to players would be opened up reasonably well.