Interstate 405 in California is the busiest freeway in the US, and among the most congested. There have been talks of building a rail line, but it would only cover the northern half (north of I-10), and so far nothing has come to fruition. Meanwhile, more lanes continue to be built, which doesn't solve the problem at all. Perhaps the best way to curb the congestion on I-405 is to build a rail line that spans the entire length (it will also be 4 tracks on the entire length). This rail line in question would be an electric heavy rail line like Caltrain electric and use the freeway median for the majority of its ROW. It would be operated by Metrolink as the 405 line. In terms of rolling stock, as is typical with Metrolink, it would use bi-level trains (EMUs in this case) though single-level trains can also be operated in off-peak hours.