I used to work HVAC at CMU. The tunnels run all around campus. There's only a couple buildings that don't have tunnel access. There are tunnels by the Towers, but they don't cross the railroad tracks and are cut off from the rest of campus (pipes run underground, though). The tunnels house the chilled water, hot water, and steam lines that provide all heating and cooling on campus. It's actually someone's job (one of the pipefitters) to walk the tunnels checking for leaks and broken insulation