Muni has once again pushed back its launch of “double berthing” — simultaneous loading of two trains in its Metro stations — until December. SFMTA spokesperson Paul Rose said that plans for a live test and launch in November were thwarted by recent weekend construction on the Sunset Tunnel.

“Because of rail improvement project work for Sunset Tunnel in November for three weekends, we had to postpone the live demo of double berthing to [the] first weekend of December,” Rose wrote in an email. “Double berthing will be released into revenue service once we run tests; we expect this sometime in December.”

When asked why work on the Sunset Tunnel would impede double berthing tests in the subway under Market Street, Rose only said they can’t happen at the same time. It’s unclear why the two conflict: The two N-Judah stops at the tunnel’s portals in Duboce Park and Cole Valley wouldn’t see double berthing, which can only be implemented at stations with platforms long enough to fit two two-car trains.

A construction project in the Sunset Tunnel has closed it to N-Judah trains (bus service runs as a substitute) over 15 weekends to replace rails, upgrade the overhead wire system, refurbish water safety valves, and install seismic safety retrofits. Outside of the tunnel, the N-Judah project will also include upgraded traffic signals with transit signal priority at nine intersections, and new wheelchair-accessible platforms at 28th Avenue and Judah Street.

The SFMTA says the weekend tunnel construction work will be suspended over the holidays, from Thanksgiving through early January, and that the entire project is expected to be finished by June.

Double berthing was originally supposed to launch in October 2012, but the SFMTA has cited difficulties in upgrading train control and signage software for the delays until this month.