入选方案是一条全地下、全自动的重型铁路线，全长约 13 miles，目标端到端行程低于 20 minutes（相较之下，开车约需 40 to 80 minutes），班距最快可达每 2.5 minutes 一班，并配备月台屏蔽门。类似的重型铁路方案在以 2023 dollars 计算时估为 $24.4 billion；更新后的成本尚未公开，但 LA Metro 仅有约 $3.5 billion 的专项地方资金，正推动公私合伙（public-private partnership），相关资料显示民间伙伴（Bechtel）可能为约 50% 的工程建设融资。更早在 2021 的采购安排了竞争路线（Bechtel 重铁 vs BYD 单轨），而 2023 的估算指出，全地下重铁选项可能比部分高架的重铁替代方案贵超过 $4 billion；同时也透过路线调整试图降低对社区的影响（包括深达约 500 feet 的路段）。
下一步受制于成本、工期与政治：在接下来 1 year，LA Metro 计划在加州的环境审查期间细化路线，并更新成本与运量预估；而为取得联邦资金所需的联邦环评流程尚未启动，且可能需要长达 2 years。该机构同时也在其 “Twenty Eight by ’28” 计划下冲刺在 2028 Summer Olympics 前交付多项工程，目前约完成 33%，领导层表示可能在 2020s 末期左右动工、在 2030s 后期通车，且可能采分段交付（先建山区路段）。在联邦优先事项变动下，资金与治理仍是关键但书，但支持者强调回报可观，包括估计乘客可为郡内每年带来约 $1.5 billion 的额外经济活动，并主张近期的网路效应可随著 D Line 延伸线以 3 phases 推进而累积需求，其第一期预计于 2026 开通。
Los Angeles’ Interstate 405 is a textbook case of induced demand: after Caltrans added 1 lane in each direction over the Sepulveda Pass in 2014 in a $1.1 billion project, rush-hour travel times were 1 minute slower 1 year later and were worse at all times of day 5 years later. The freeway now carries about 400,000 vehicle trips per day, often at near walking pace. Against that backdrop, in January the LA Metro board approved the outline of the Sepulveda Transit Corridor: a new subway roughly paralleling the 405 through the Santa Monica Mountains, with projected ridership of more than 100,000 passengers per day and a development timeline of a decade or more.
The selected concept is an all-underground, automated heavy-rail line roughly 13 miles long, aiming for an end-to-end trip under 20 minutes versus about 40 to 80 minutes by car, with trains as frequent as every 2.5 minutes and platform screen doors. A similar heavy-rail concept was estimated at $24.4 billion in 2023 dollars; updated costs are not yet public, but LA Metro has only about $3.5 billion in dedicated local funds and is pursuing a public-private partnership, with materials indicating a private partner (Bechtel) could finance roughly 50% of construction. Earlier procurement in 2021 set up competing approaches (Bechtel heavy rail vs BYD monorail), and 2023 estimates cited that a fully underground heavy-rail option could be more than $4 billion costlier than a partly above-ground heavy-rail alternative, while route changes sought to reduce neighborhood impacts (including segments as deep as about 500 feet).
Next steps are cost-, schedule-, and politics-constrained: over the next 1 year LA Metro plans to refine the route and update cost and ridership estimates during California environmental review, while the federal environmental process needed for federal dollars has not started and could take up to 2 years. The agency is simultaneously racing to deliver projects for the 2028 Summer Olympics under its “Twenty Eight by ’28” program, with about 33% completed, and leadership suggests groundbreaking could occur toward the end of the 2020s with opening in the late 2030s, potentially via phased delivery (mountain segment first). Funding and governance remain key caveats amid shifting federal priorities, but proponents cite large returns, including an estimate of about $1.5 billion per year in additional county economic activity from riders, and argue near-term network effects could build demand as the D Line extension advances in 3 phases with its first phase expected to open in 2026.