Complex sentence: The train system is located within the secure area of the airport, with four stations serving each airside concourse (Concourses A, B, and C) and the Main Terminal (which serves Baggage Claim and Ground Transportation).
Split into simpler sentences: The train system is located within the secure area of the airport. The system consists of four stations serving each airside concourse (Concourses A, B, and C) and the Main Terminal (serving Baggage Claim and Ground Transportation).