Simplify the following text for a Grade 11 native English-speaking person.
Original: The Center City Commuter Connection , commonly referred to as `` the commuter tunnel '' , is a passenger railroad tunnel in Center City , Philadelphia , Pennsylvania , built to connect the stub ends of the two separate regional commuter rail systems , originally operated by two rival railroad companies : the Pennsylvania Railroad and the Reading Railroad .
Simplified: The stations connect the networks of rail previously operated by the Reading Railroad and Pennsylvania Railroad .