One v real and v technical barrier to regional rail implementation in the US is gonna be track access. Hourly (or less) off-peak and weekend service allows single tracking/long work windows => a lotta weekday midday work.

Many examples of this across the US:
- LIRR schedules allow single tracking through the 3rd track area
- NJT has been scheduling weekend service to run only in one of the two Hudson River tubes since ~forever
- MBTA Worcester schedule allows single tracking west of Framingham

15 min all-day frequencies eliminates those windows, forcing all work overnight. Especially if you treat daytime service as 'sacred' (ie an absolute minimum of service reductions/modifications) this has a few implications:

- Your maintenance productivity has to rise considerably to keep up with existing maintenance burden + increment imposed by higher infra use
- The 'run freight at night' fix for passenger-freight interactions may not be (as) viable

- You may be forced to revisit roadway worker protection rules/protocols to allow some basic maintenance to take place under traffic without disrupting schedules

These are by no means insurmountable challenges. Just things important to recognize in planning regional rail. Investments in maintenance capacity are paramount to success.