The San Diego Association of Governments (SANDAG) in the US has started the first phase of a double-track rail project between Sorrento and Miramar in California.
The construction is part of an $800m rail improvement programme along San Diego County's 60-mile segment of the Los Angeles-San Diego-San Luis Obispo rail corridor (LOSSAN).
SANDAG chairman and Encinitas Mayor Jerome Stocks said: "Building on Amtrak's recent completion of two miles of double tracking in Carlsbad, we will have three new miles of double track completed in three years, which will help reduce travel time and accommodate the growing demand for passenger and freight services in the region's North Coast Corridor."
The Sorrento to Miramar double track project includes two phases of construction designed to help speed up passenger and freight trains on the corridor by straightening the steepest curves.
Scheduled to be completed by the end of 2013, phase 1 will include the construction of 1.1 miles of a second track, parallel to the existing line.
Under the first phase, a wooden trestle bridge built in the 1940s will be replaced, retaining walls will be added and a new track crossover will be installed south of Sorrento Valley Boulevard.
Phase 2 of the project, which is in the design and environmental review stages, will see the addition of another 2.1 miles of double track and straightening curves.
SANDAG said that the current stretch of single track causes a major block to rail traffic because sharp curves restrict train speeds to 25mph.
The first phase of construction is expected to cost $43m, $29m of which will be covered by sales tax for transportation.
Design and construction of the two phases will be managed by SANDAG along with the San Diego Metropolitan Transit System, which owns the rail line south of Del Mar, and the North County Transit District, which manages, operates and maintains the track.
Over the next 20 years, the association aims to invest about $800m in the county's portion of LOSSAN to increase capacity and ensure reliability and safety of intercity, commuter, and freight rail services, according to Railway Technology.