Which agencies are involved exactly? Apparently a lot of them are involved in this type of thing according to TBC:
- Federal Highway Administration
- Federal Railroad Administration
- National Park Service
- US Army Corps of Engineers
- National Capital Planning Commission
- US Environmental Protection Agency
- DC Department of Transportation
- Baltimore City
- Maryland Department of Transportation
- Others - many federal/state/local agencies and bodies
While the Loop does not draw a vacuum, it's tunnels are apparently compatible with Hyperloop requirements, meaning a future expansion could mean an upgrade one day to full blown Hyperloop capability.
The tunnels would be at least 30 feet deep, 12-24 feet in diameter and construction would take approximately 1-2 years if approved.