I checked via "Search" to see if this has been previously posted. Seems it hasn't:
Astronauts To Ride Rails In Emergency
it is fundamentally a group of multi-passenger cars on a set of rails reminiscent of a roller coaster. Its purpose is to move astronauts and ground crew quickly from the vehicle entry on the launch pad to a protective concrete bunker in case of an emergencyKelli Maloney, the lead designer for the launch pad escape system, said a trade study showed the railcar best met NASA's requirements. Those requirements call for astronauts to be able to get out of the spacecraft and into the bunker within 4 minutes.
One of the benefits of the rail system, Maloney said, is that the track can take the astronauts directly to the bunker door. That would be a big help if one of the crew members or a ground crew member was incapacitated.