Mid-Coast Trolley Project
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The Mid-Coast extension runs alongside the tracks used by the Coaster commuter trains and the Amtrak Pacific Surfliner, except at the northern end, where it is planned to climb Miramar Hill to serve UCSD; the mainline tracks go around the hill instead. Right now, the plan is to build dedicated tracks for the trolley; if it were a tram-train, it could instead use the existing tracks shared with regular trains and only use new tracks to climb the hill to UCSD.
Tram-trains also have a history in the U.S., by another name: the interurban. In the first half of the 20th century, private companies built long commuter lines and even intercity rail lines. They were mostly separate from the conventional rail network, and were electrified rather than steam-powered, but ran like regular railroads between cities; within cities they ran on city streets, like streetcars. At the beginning of the 20th century, there was some track-sharing between urban transit and commuter rail in New York, but federal regulations outlawed it in the 1910s on safety grounds.
Today, some former interurbans have been turned over to light rail, which runs on city streets in the center and on dedicated railroad lines farther out. Unfortunately, the need to construct new dedicated tracks for every light-rail line, rather than sharing regular train tracks, leads to high construction costs. Using the existing rail tracks is cheaper, and San Diego should investigate if it is possible to modify the Mid-Coast extension to do so.
There is already one place in the U.S. using a proper tram-train using mainline rail: the New Jersey
River Line uses city streets in Trenton and Camden and mainline track between them. The regulations for this are complicated, and River Line has strict time separation, with freight trains running only at night. But European regulations are friendlier to tram-trains. An ongoing overhaul of American passenger rail regulations is planned to permit lightly modified European trains on American tracks, and it is likely that sharing tracks between intercity passenger and freight trains and light rail will soon be permitted.
Public transit consultant Jarrett Walker
discussed tram-trains in 2009. He said that the technology is useful when there is a dominant destination that is a little too far away from the train tracks. In the case of San Diego, this is UCSD. Walker cautioned that street-running slows down the trains, since they have to obey street traffic rules; in this case this is not a problem, since the segment through UCSD is planned to be elevated rather than on the street.[...]
