Saturday is the soft opening of Muni's long-awaited, much-delayed Central Subway, with free weekend service beginning to just the four new stops on the new line. It's January when the big service changes begin.

Yes, I can hardly believe it either, but the Central Subway will actually begin welcoming passengers in its tunnels and platforms on Saturday, November 19. This soft-opening trial period, as previously reported, is just a weekend deal, and apparently a way to work out some kinks before the new line gets hooked up and merged with the T-Third, which will mean that you won't see T trains in the regular downtown Muni tunnels anymore come January 7.

As the Chronicle explains, and as Muni will be reminding you the next two months, the Central Subway will become a new arm of the T-Third line, taking passengers all the way from Sunnydale to Chinatown, should they wish to take that trip. T-Third passengers coming from elsewhere in the city on other train lines will, starting January 7, have to transfer at Powell Street/Union Square, and vice versa. As shown on the updated Muni map below, the T no longer will take the trip through the original-flavor Muni tunnel and up onto the Embarcadero.

Map via SFMTA

Instead, the N-Judah will become the new way to get from the center of the city to Oracle Park — you'll need to transfer to the N if you're coming from, say, Noe Valley, the Castro, or Balboa Park and heading to a game or concert. The K-Ingleside, which for years has taken people along the Embarcadero to the ballpark, will then terminate at Embarcadero, and those passengers will need to transfer to the N if they need to go further. Or, alternately, passengers can transfer to the T-Third at the new Central Subway stop at Union Square, and ride the train to 4th and King that way.

Big changes! But, for now, through the holidays and into the first week of January, the T-Third will keep running its usual route, out along the Embarcadero and turning left after the ballpark to head out to the Bayview.

And your chance to check out the Central Subway will be from 8 a.m. to midnight, just on Saturdays and Sundays for the next seven weeks.

Previously: T-Minus Five Days! Central Subway Trains Set To Start Rolling This Saturday