is there a link to all this infrastructure I didn't know existed.
https://en.m.wikipedia.org/wiki/United_States_Capitol_subway_system
That's a pretty good overview. It's not a massive secret system by any means. It's been there for years. Capitol Hill, once you put in the three House office buildings, the two Senate office buildings, and the Library of Congress is a massive area and the subway and tunnels underground the complex allow visitors, staff, and Members of Congress to easily go from one side of the capitol to the other. With the system it can take half an hour to go from the House office buildings to the senate. It could take an hour with having to wait on traffic and to navigate the complex above ground.
The system also allows for members, staff, and visitors to stay within the security perimeter without having to be re-searched every time they enter and exit a building, assuming they use the underground system and don't go outside to street level.