Local political leaders and transportation officials took part in the groundbreaking ceremony in Santa Fe Springs.
The $110-million project will widen the freeway as it passes the Alondra Boulevard overpass.
One carpool and one traffic lane will be added in each direction from North Fork Coyote Creek to Marquardt Avenue.
Traffic will be impacted during the construction period and Metrolink officials are encouraging commuters to take public transportation.
"We need to have freeways and highways and overpasses like this and carpool lanes, but we also have to make sure the trains and buses light, rail and heavy rail are all part of the solution," said Richard Katz, Metrolink Board chairman.
Construction on widening the freeway is expected to begin next month and be completed by late 2015.