On Saturday, BART fans thronged Lake Merritt Station to attend a big birthday bash for BART's 50th birthday. It was a day filled with themed cakes, a time capsule unveiling, and a "downright disturbing" train replica with a human-like face.

When service began on BART on September 11, 1972, the Bay Area looked quite a bit different. There were around 4.65 million residents of the region — three million less than there are today. San Francisco's rent control ordinance hadn't yet passed. And BART is yet to experience the online fanfare it would eventually garner.

Fast forward 50 years, BART has now become the fifth-busiest heavy rail rapid transit system in the United States — serving about 400,000 riders a day during the workweek— expanded to 50 stations (some of which finally have open public bathrooms, again), and grown into something of a Twitter juggernaut.

Much like Jennifer Lopez, BART has proven age is nothing but a number... and that you can continue peaking and thriving and winning well past your bemused 20s and early 30s. This past Saturday, thousands of people convened to celebrate the Bay Area's beloved rapid transit agency's 50th birthday.

It was an afternoon filled with excitement and moments of gratitude. A seated crowd underneath a marquee tent, quite literally, sang "Happy Birthday" to the inanimate network of railways and trains. There, of course, was cake served that was assembled and frosted to look like BART's fleet.

Booths — which focused on transit-oriented development, accessibility, and sustainability, as well as homages to BART's history, like the one set up by the Western Railway Museum — were set up to engage attendees, alongside games for small humans to play. A portion of a track was designated for people to interact with; one retired regional transportation planner "finally [touched] the third rail."

The 1992 silver time capsule that was recently dug up from Lake Merritt was opened for the 50th-anniversary event, which was unveiled in tandem with BART revealing what was going in its new larger one that would also be buried in the Oakland lake.

And yes: You can't unsee this creepy-looking replica of a BART train put on display, which also made an appearance during the Warriors' victory parade in June.

All in all, it was a big day for fans of the public transit agency — one that you can relive vicariously through our favorite tweets about the event below, if you weren't able to attend or hop on the livestream.

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Photo: Twitter via @rideact