West Coast Rail Constructors (WCRC), a joint venture partnership between Fluor and Flatiron, was selected by the North County Transit District (NCTD) to convert approximately 22 miles of active freight rail between Oceanside and Escondido, California to a new commuter rail service. NCTD's Sprinter Mainline project offered alternative passenger transportation to one of the region's most heavily traveled corridors. The daily capacity of 452 passengers carried on 64 trips per day at 55 miles per hour reduced highway traffic and pollution in North San Diego County.

Construction included new ballasted track, signaling systems, bridge and culvert structures, 15 new passenger transit station platforms, communications and controls systems, retaining walls and landscaping. The $224 million Sprinter Mainline and the San Marcos Loop opened northern San Diego County to light rail transit. The San Marcos Loop was completed in July of 2007. The Mainline Project completed in December of 2007.