Construction is expected to begin by the end of the summer on the 39km line from Riverside Downtown to South Perris, although a $US 75m federal grant still requires approval and the lawsuit must be formally withdrawn.

The group, Friends of Riverside's Hills, told local media it has obtained concessions worth about $US 3m, involving noise mitigation measures, footpaths, and biking facilities.

Last May a judge rejected the environmental impact report (EIR) for the proposed line, ruling that the Riverside County Transportation Commission had failed to address issues such as wheel noise and pedestrian safety.

The line will operate as an extension of Los Angeles Union – Fullerton – Riverside service.

The extension will use the alignment of a former freight line, and the responsibility for maintaining the infrastructure will pass to the Southern California Regional Rail Authority (SCRRA), the public agency that operates Metrolink.