The stretch from Los Angeles to San Diego is one of America’s busiest travel corridors. The I-5 is among America’s busiest interstates, and Los Angeles-San Diego is the top corridor for high-speed rail in California, according to the America 2050 report by New York-based Regional Plan Association. The Pacific Surfliner, the line that connects San Diego and L.A. and goes north to Santa Barbara, is already the second busiest Amtrak route, after the Northeast Corridor.

Yet the plans for California’s high-speed rail prioritizes the route from Los Angeles to San Francisco instead. It will take decades for high-speed rail service to reach San Diego. There are steps Southern California officials could take in the meantime, however, that would drastically improve rail services and encourage more people to ride.

Existing upgrade plans leave something to be desired – they’re both low-cost and low-impact. These include some track upgrades that would let trains travel faster and more frequently. Several additional projects are part of the California high-speed rail program. The so-called blended plan involves incremental improvements to track speed and capacity between Los Angeles and Anaheim, especially on a short segment with heavy freight traffic. This is intended to allow future high-speed rail from Los Angeles to San Francisco to use the existing tracks to serve Anaheim, but at lower speed. The total cost on this segment is projected at $2 billion in the 2016 business plan.

Phase One of California high-speed rail, between San Francisco and Anaheim, will only open in 2029, and the High-Speed Rail Authority has so far done little work on Phase Two, which includes the line between Los Angeles and San Diego, via the Inland Empire. Since high-speed rail service to San Diego is so far on the horizon, it is worth discussing medium-term improvements, which would take several years instead and upgrade service before high-speed rail arrives.

Examples of these interim improvements already exist. For instance, the Northeast Corridor – the East Coast rail line that runs from Boston to Washington D.C. – has been improved slowly over many decades, is electrified and runs at an average speed of 60 to 80 miles per hour. Some European countries, including Britain, Sweden and Switzerland, have not built high-speed networks but instead upgraded legacy lines. In those countries, upgraded lines average between 70 and 90 miles per hour, supporting multiple trains per hour on the busier lines. San Diego is bigger than any Swedish or Swiss city, and the five-county Los Angeles metro area is bigger than Sweden and Switzerland combined. If domestic trains in Sweden and Switzerland can support one to two trains every hour, fast service between Los Angeles and San Diego should support at a minimum a train every half hour, and potentially much more.

The Los Angeles-San Diego corridor is 128 miles long, and is for the most part straight. Target trip times of two hours should be achievable even with the frequent stops on the Pacific Surfliner. The aspirational trip time is about 1:45 or 1:50, which would be competitive with driving even outside rush hour. The investment required for this ranges from the high hundreds of millions of dollars to the very low billions. This is still slower than the eventual trip time envisioned by Phase Two of the high-speed rail project, currently projected at 1:18, via an indirect route through the Inland Empire.

The way to achieve trip times lower than two hours on legacy track is to combine new federal regulations and strategic investments intended to take advantage of the new rules. In late 2016, the Federal Railroad Administration released new regulations for passenger rail safety, which allow lightly modified European trains to run on U.S. tracks. Previously, unique U.S. rules required trains to be heavier. This follows a regulatory change from 2010 that allows trains to run faster on curves, subject to safety testing. The existing diesel locomotives are too heavy to take advantage of this change, but lighter electric passenger trains face no such obstacle.

This means that the region needs to invest in electrifying the corridor from San Diego to Los Angeles, and potentially as far north as San Luis Obispo. Between San Diego and Los Angeles, the likely cost – based on the California high-speed rail electrification cost – is about $800 million.

The benefits are considerable. Electric trains emit no local pollution, while diesel is an unusually dirty fuel, contributing to Southern California’s poor air quality. New EPA rules, the Tier 4 standards, have required rail agencies in the U.S. to buy cleaner-burning diesel locomotives. The Pacific Surfliner has recently bought Tier 4-compliant locomotives, but many intercity and commuter rail routes around the country are interested in such trains, so they could likely fetch a good price by selling them now on the second-hand market. While these locomotives are cleaner than the legacy ones they replace, they are almost as heavy, and are unsuitable for a fast operation.

Besides the environmental benefits, electric trains have far better acceleration than diesel trains. An analysis by local rail activist Paul Druce suggests that on the northern half of the line, a European electric commuter train could average about 60 mph, making many stops on the way. This adds to the ability of such trains to go somewhat faster on curves without compromising safety. The existing plan for the corridor already includes some speed increases; being able to run faster on curves would have a noticeable effect. Better rail transit coming from electrification would have additional environmental benefits coming from reducing driving, such as car accidents, pollution and congestion.

Another potential investment is a cutoff of Miramar Hill. Right now, the tracks meander on a curvy alignment, wasting valuable time. Local medium-term plans for the corridor include a tunnel under the hills, which would shave about 4.5 curvy miles off the route, saving perhaps seven minutes. The projected cost of the tunnel is about $500 million. But electric trains can climb steeper grades than diesels because of their more powerful motors. They could run on new tracks alongside I-5, with some viaducts but no tunnels.

All of the above improvements work together. New regulations allow the corridor to use more powerful trains. This encourages electrification, in order to immediately buy the best standard-speed trains available, and run faster on curves. Electrification, in turn, encourages a cheaper Miramar Hill realignment than the proposed tunnel.

The result of such investment would be that frequent, rapid trains could efficiently connect Southern California by the mid-2020s. With trains doing the trip between Los Angeles and San Diego in less than two hours, many people would choose to leave their cars at home and ride rail. Trains would leave every half hour, all day, every day. Travelers could connect from anywhere on the San Diego Trolley system to anywhere on the expanding Los Angeles Metro Rail without using road-based transportation. This would not only shift travel away from highways and toward mass transit, but also encourage taking more trips, for tourism as well as business. The Los Angeles and San Diego metro areas would become closer and better-integrated.

Alon Levy is a Paris-based mathematician and public transportation policy writer.