20 U.S. cities have commuter rail lines; others are talking about opening commuter rail. Should they? The best way to answer that question is to figure out what commuter rail does well. Those other cities can give us a good idea of why successful commuter rail systems are successful.

Here are eight criteria for the perfect commuter rail line. Only a few lines meet all of these. But nine of the top 10 commuter rail systems in the United States meet at least seven.

1. The ideal commuter rail line improves on current transit options.

This seems obvious, but it’s worth remembering that transit on steel wheels is not automatically better than transit on rubber tires. Commuter trains are big, so in order to fill them they can’t run more frequently than every half hour during rush hour and maybe every hour during midday, and they need to stop at multiple stations to fill up. Also, unless there happens to be an existing rail line right in the middle of an employment center, most riders will need to transfer to get to work. All of that is worth it it your other option is buses stuck in freeway traffic, but not if you could take a reliable non-stop bus that uses reserved lanes.

2. The ideal commuter rail line makes use of unused rail capacity in a corridor where highway capacity is scarce.

The way to implement commuter rail quickly and cheaply is to find a freight rail line that isn’t heavily used, so only relatively minor upgrades are needed to get trains running. Where freight rail lines are already busy, adding passenger trains means adding tracks, and that can get expensive, particularly when the right of way the tracks are in is narrow.

3. The ideal commuter rail line serves more than commuters.

Rush hour only service is not an effective use of equipment. On some lines, each set of locomotive and cars makes only one trip inbound in the morning and one trip outbound at night. That’s not much benefit from a $9 million piece of equipment. A route that has ridership during the day, in late evening, and on weekends will get more use out of the same equipment and infrastructure.

4. The ideal commuter rail line has a city at each end.

If there’s an employment center at each end of the line, you’re serving two rush hour commuter flows. You’re also serving a whole other kind of trip: people traveling from one city to another.

5. The ideal commuter rail line offers good connections to multiple employment centers.

Not everyone works Downtown. For example, Houston has least four other major employment centers in the urban core: the Medical Center, Uptown, Greenway, and UH. People who work in those places should have access to good transit, too.

6. The ideal commuter rail line serves long trips.

The biggest advantage of trains over HOV or HOT lanes buses is comfort. A train has a smoother ride, wider seats, and the ability to get up during the trip. Commuter trains can also offer work tables, power outlets, wifi, restrooms, and an onboard coffee counter. These things don’t matter on a 20-minute trip. But they really make a difference on a 1-hour trip.

7. The ideal commuter rail line connects to local transit.

Commuter rail can only go where railroad lines are. That means most riders will need to connect to another mode of transit on at least one end of their trip. To For connections to major activity centers, that needs to be high quality, frequent service, not just local buses. But it’s also important to have transit service in other places, to allow those who can’t drive to get to the stations.

8. The ideal commuter rail line has stations you can walk (or bike) to.

Transit is inherently pedestrian-oriented: there’s no way to have a car waiting for you at both ends of your trip. Having a car waiting at one end — park-and-ride — works, but it’s inherently inefficient, not only for the transit system that needs to provide 200 square feet of pavement for each passenger but also for the passenger who still needs a car to use the system. Putting stations where people can walk to them — not just at employment centers but also in suburban communities — works better for everyone. You still provide parking lots, but you don’t need to provide them for everyone.

Some examples:

The most successful commuter rail systems in the United States — New York, Chicago, Philadephia, Boston — are over 100 years old. They meet all of these criteria. They have an unfair advantage, though, because cities and towns grew around them. The Downtown stations are really Downtown; the suburban stations are in small town downtowns; and the systems use high-quality infrastructure — often separated from freight rail — that dates to the 1920s or earlier.

So let’s consider more recent successes. Caltrain stands out. It’s been around for a long time, too, but it’s experienced dramatic ridership growth in recent years: 18,000 in 1985, 32,800 today. Why? It connects two major employment centers — San Francisco and Silicon Valley — so northbound and southbound passenger flows are nearly matched. It stops in lots of places where people want to go: small town downtowns with stores and housing, Stanford University, San Francisco airport. That creates all day demand, and Caltrain meets it with all day service: first train at 4:30, last train at 10:30, and trains at least every at least every 30 minutes. It connects at 5 stations to 3 different urban rail transit systems. And — last but not least — it parallels an extremely congested highway on a double- and triple- track rail line with virtually no freight traffic.

Another success is Virginia Railway Express, which connects Northern Virginia suburbs to Washington D.C.. It’s blessed with excellent station locations in the suburban office centers of Alexandria and Crystal City and two stations right in the Downtown core. Many of its suburban stations are in small town downtowns. It also has excellent connections to the subway system (at four stations) and comprehensive local feeder bus service.

The biggest commuter rail system implemented in the United States since World War II is in Los Angeles. MetroLink has seven lines, generally shared with freight rail but considerably upgraded and improved. It’s a truly regional system, with lines reaching as far as 90 miles from Downtown LA, connecting to cities like San Bernadino, Anaheim, and Oceanside. It also has local transit connections: the vast majority of MetroLink riders transfer to the Red Line subway to get from Union Station into the Downtown office core. MetroLink also connects to another commuter rail system — San Diego’s Coaster — and to frequent Amtrak service to San Diego and Santa Barbara. MetroLink’s biggest asset, though, is incredibly congested highways.