San Francisco had an extensive trolley system involving two companies--Market Street Railway, which was private, and the San Francisco Municipal Railway which was publicly owned. MSR's lines were numbered and Muni's were lettered. Eventually the system came down to just five lines left, all Muni: J, K, L, M and N. This contraction was occurring during a time of transitioning to one man operation, which was resisted, and the increasing postwar popularity of the automobile, mentnioned earlier in these posts. What also helped JKLMN to survive was private ROW's and a couple of very long tunnels which enabled KLMN to avoid SF's famous hills. The J-Church ROW was itself built over a hill near Mission Dolores. They have just converted a long bus route, the 15-Third, to light rail. Since they are using the Muni letters for rail, It is now the T-Third. I wonder about that; maybe keeping it as the 15 would have been better.

Please see:
[www.streetcar.org]
for much more on the SF historical context of this topic.