Tokyo has two railway lines which loop around the central area of the city. The more famous of these is the Yamanote Line, the inner loop which connects all of the key commuter rail terminals. The Yamanote is an old line—dating back to the late 19th century and completed in the early 20th—and thanks to its central location and unique routing has ridiculously high ridership, with trains running every two minutes on weekdays, and seats which retract during the morning rush hour to provide extra standing room.

Then there is the Musashino Line. It is located farther out in the suburbs, running through places like Tokorozawa (the 1950s version of which is the bucolic setting of My Neighbor Totoro) and the equally unexciting city of Funabashi. There are few major commercial centers along the line's route, and ridership is quite low. Despite this, the line supports passenger trains every half hour or so. And its construction came at obvious expense—much of it runs underground, the sort of construction undertaking that is normally only seen on inner-city corridors and intracity trunk lines, including a lengthy segment at its southern end which doesn't even have passenger service. What gives?

As far back as the 1920s, there were plans to build an "outer loop line" around Tokyo for the use of freight trains. Execution of the plan did not begin until the early 1960s, and even at that point it was a fairly low priority, as the existing inner tracks of the Yamanote Line were enough to meet the demand for rail freight at the time.

After World War II, the United States Armed Forces had taken over the Japanese military airbases in the western suburbs of Tokyo. Some (Yokota and Atsugi) are still operating, while others (Fuchu, Tachikawa and what is now Haneda Airport) have since shut down. But these bases all required a steady supply of JP-4 to fuel the various aircraft stationed there. Then, as now, the US military had its own logistics facility on the coast of Kawasaki where jet fuel would be loaded onto trains bound for the various bases outside Tokyo. Initially, the trains headed for bases in west Tokyo would go up the Tokaido Main Line toward Osaki, then up the inner tracks of the Yamanote Line to Shinjuku, then outbound on the Chuo Line to their final destination.

Then, early in the morning on August 8, 1967, a freight train carrying fuel bound for Tachikawa collided with another freight train while passing through Shinjuku. The resulting fire took over 24 hours to clean up (even with US military assistance) and shut down the Chuo Line for a day, affecting two million commuters. Since the fuel trains had always run through Tokyo overnight, they had never been noticed as a potential safety hazard: this incident, however, changed public and official opinion on the matter, and expedited the plans to complete the outer loop line.

It was an inconvenient time for public works, though. While the Musashino Line was being scribbled onto the drawing board, residents on the other side of Tokyo were in the midst of violent protests against the construction of Narita International Airport, where eminent domain proceedings were met by angry farmers chaining themselves to trees and frothing left-wing student activists screaming conspiracy theories about Japan becoming the accessory of the US against the great socialist revolution. So when the Japanese National Railways (the predecessor of today's JR companies) was faced with the task of seizing land on the other side of town to build a freight railway for American jet fuel, they knew they had their work cut out for them.

JNR and the locals reached a decent compromise, though. In exchange for the land to build the new line, JNR agreed to provide regular passenger service along the northernmost 60 km. The remaining 40 km at the south end, running mostly underground through areas of Kawasaki which were already developing as bedroom towns for central Tokyo, remained a freight-only line except for certain special through service by intercity express trains. The passenger area opened for service in 1973: the underground freight extension through Kawasaki was completed in 1976.

That said, the Musashino Line is not a complete loop line around Tokyo, although it does come close. The loop is completed by the Tokaido Line (from Tsurumi to Osaki), the Rinkai Line (from Osaki to Shin-Kiba) and the Keiyo Line (from Shin-Kiba to Nishi-Funabashi). Although the latter lines were originally contemplated as a freight route between Chiba and western Japan, the Rinkai Line ended up developing as a passenger line, in large part due to the unexpected development of the Odaiba waterfront area as a commercial and recreation center.

The Musashino Line itself has not even become as much of a freight line as was originally anticipated: it carries more passenger trains than freight trains, and in the city of Saitama it suffers severe undercapacity for passengers during rush hour, when trains are reportedly carrying twice as many passengers as they are designed to accommodate. Part of this is due to the improvement of expressways around Tokyo, which has made trucks more efficient for shipping things around. The eastern side of the line (between Matsudo and Funabashi) has no freight service at all.

Some quirks of the Musashino Line:

  1. Passenger trains on the line run between Fuchu-Honmachi (in the city of Fuchu on the west side of town) and Tokyo Station, the latter of which is reached by switching over to the Keiyo Line at Nishi-Funabashi. Although all the clockwise trains are "bound for Tokyo," they are almost never the most convenient trains for reaching Tokyo (unless you are boarding past Matsudo), as Tokyo lies right in the middle of the loop and is much more quickly accessed on one of the "spoke" lines such as the Chuo Line from Fuchu and the Keihin-Tohoku Line from Saitama. Some stations warn passengers of this fact in announcements.
  2. The "starting point" of the Musashino Line is actually in Kawasaki, on the far end of the line from Tokyo Station. Therefore, the trains going toward Tokyo Station are labeled as "outbound" (kudari) in timetables, which often confuses the handful of people who still use timetables.
  3. The Musashino Line was the first JNR line to use the magnetic ticket-reading automated station gates which are ubiquitous throughout Japan today. This was in part because it was built at an appropriate time to be a test subject, in part because JNR did not want to staff many of the more rural stations on the line, and in part because the anticipated low passenger volume made it easier for JNR to sort out bugs in the system (there were many at first).

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