In the campaign for Measure M in 2016, Metro introduced a plan to build a two-pronged extension of the Eastside branch of the L Line. Six years later, it seems that Los Angeles County may have to settle a phased build out of just one.

Faced with multi-billion dollar funding shortfall, Metro staff have recommended that the Eastside extension be built in phases, starting with a 4.6-mile initial operating segment running to Greenwood Avenue in the City of Montebello. The first phase of the project would start running in a subway alignment below Atlantic Boulevard, with stops at Whittier Boulevard and Commerce. After that point, the alignment would daylight to at-grade and aerial tracks running along Washington Boulevard toward a new maintenance facility and temporary terminus to the west of the Rio Hondo.

Even the initial segment may represent a heavy lift, based on existing funding streams. While the project's estimate cost is nearly $5.9 billion based on 2022 conditions, the staff report estimates that inflation could push the price tag up to $7.9 billion during the course of construction. State resources could supplement future sales tax returns that have already been allocated to the project - estimated at $4.4 billion - leaving Metro $3.5 billion short of what the project may require.

To make up for the shortfall, staff have also recommended that the project should go through a review under the National Environmental Policy Act, which could set up the Eastside extension to receive Federal grant funding - potentially through the $1-trillion infrastructure bill passed by Congress in 2021. The extra funding would help cover the cost of more expensive elements, notably the extensive below-grade tracks and new maintenance and storage facility.

While the initial operating segment would hit key destinations, including the busy commercial hub along Whittier and the Citadel in Commerce (which is planning its own monorail as part of an expansion), it remains to be seen what will happen with the remainder of the extension - which would stretch an additional four miles and have stops at Rosemead Boulevard, Norwalk Boulevard, and a new terminus at Lambert Road in Whittier. That final extension, planned as a subway, could push the total cost of the Eastside extension to more than $10 billion.

Plans for transit in the San Gabriel Valley and other communities in the eastern side of L.A. County have undergone numerous changes since the passing of Measure M in 2016. Plans for a two-pronged L Line extension were pruned in 2020, when Metro dropped a proposal to build a second alignment parallel to the SR-60 Freeway, citing both the high cost and the operational complexities that would be created. That move freed up more than $600 million in funds which will likely be reallocated toward the creation of new bus rapid transit lines along Valley and Rosemead Boulevards.

This is not the first Metro Rail expansion to recently be pared back as a consequence of ballooning costs. The Metro Board has also voted to split a new light rail line along the Van Nuys Boulevard corridor into phases, as well as the proposed West Sana Ana Branch.