Nxsys, Signalling and Interlocking Simulator

Also see:

- https://bernardgreenberg.com/Subway/

- https://github.com/BernardGreenberg/NXSYS

- and the incredible pages on the nycsubway.org site such as https://www.nycsubway.org/wiki/Subway_Signals:_A_Complete_Gu...

- and https://bernardgreenberg.com/Subway/bsg-subway.html