When San Franicsco’s Central Subway opens in 2019 it will have been a full thirty years from the time voters approved first the four-corridors plan in 1989. Together the subway and surface section of of the T-Third Street Muni Metro line are two of the corridors, dedicated Muni busways Van Ness Avenue and Geary Boulevard are the other two.
The Muni Metro light-rail (shown in orange) opened in 1980 with five former trolley lines converging into the upper level of the Market Street Subway (BART runs on the lower level and opened in 1973) and extended in 1995 on the surface along the waterfront to AT&T Park (Go Giants!) and Caltrain. The lines were configured so the JKLM lines ended at the last subway station at waterfront and the N-Judah alone traveled along the surface.
In 2007 the first phase of the T-Third Street light-rail line (red) opened. The subway section was still at least a decade off so the new line running up the eastern waterfront was connected up with the existing trackway running into the Market Street Subway. This turned out to be a disaster that had delays upwards of 20 minutes on a daily basis for the first six months.
T-line trains were to turn-back at Castro Station, while the rest of the lines continued on. Without a third track for T-line trains to layover on, trains had to switch directly from the outbound to inbound track before it could board and heading back. Switching over blocked traffic in both directions, having to wait for the track to clear made it even worse. This created an unavoidable delay a few minutes long, every 8-10 minutes.
After six months the whole thing was rejiggered. T-Line trains no longer turned around at Castro and the solution was just to extend K-Ingleside line the entire way down Third Street. Trains change name entering the tunnel at either end which created the current KT-Ingleside/Third Street.
The turn-back at Castro was only one of the problems, Muni still doesn’t have enough vehicles (only last week did Muni order more trains for the T-Line) and the 90 degree turn at by the ballpark is still creating an extra signal phase that stops car and rail traffic so KT trains can make the turn.
In 2019 the Central Subway is planned to open and the T-line will no longer make that ninety degree turn. They will instead continue north, entering a tunnel to Chinatown.
At that point the K-Ingleside will go back to stopping at the Embarcadero subway station (along with the JLM lines) and the N-Judah trains running along the waterfront will cross the T at a right angle. The lines have separate platforms in the medians and trains will be able to with car traffic (and without extra signal phases) so neither one will be getting in the other’s way. With the new trains that takes care of the three biggest delay sources of delays on the KT and N lines.
Even the naming starts making more sense; the consecutively lettered JKLMN lines will run through a different subway than the T line runs through.