The Brotherhood of the Bike originally uploaded by steelmonkey
The bike car on the Caltrain trains is at one end of the train. That is, there is only one place on the train for bikes [as far as I could tell], and its an entire car. Thus, if you want to bring your bike on the train, it's likely you will be in this car (to remain close to your bike, to get to it easily & quickly when the train stops at a station). It's also likely then, if you are a frequent/daily rider, that you will tend to see the same people repeatedly. And, like you, they will also be riders.
bike car. caltrain originally uploaded by luckyklover
Which means you will probably identify with them. And you will have stories to share. You will experience aspects of the day in similar ways ("oh man, that can't have been good in the rain!"). You will trade hacks, fixes, workarounds, secrets. You will commiserate, and look for each other, and wonder when people are missing. Some of you will be more gregarious, popular, visible, more contributing than others. There will be quiet, morose types. There will be lurkers - people who never become visible, but who participate nevertheless, just by being present and taking it in. After a while, you will look forward to being in the bike car.
In short, you will probably form a tribe, after a fashion. (I'm speculating, of course. Evidence is lacking. Would you like to get me some?)
Now there is something peculiar about this tribe. It is created by the architecture of the train. It is contingent upon just the right people coming together in the same place, but not in a manner of their own design or intention (perhaps we can call them 'tribes of contingence'?). This tribe will, if it forms, have been created due to policymakers and the operations people at Caltrain. It will be the outcome of decisions made on entirely other grounds: efficiency, safety, comfort. But lo and behold, decisions made in boardrooms and committees create this cluster of people who find themselves having something in common with each other.
This is the opposite of Meetup. Here, you did not have to seek out others of your kind. The world architected your meeting. You didn't plan it, you came into it.
This is something technology can do really well, but has only taken hesitant steps into. It does not, as of yet, create coincidences (without your express effort - else it wouldn't be a coincidence) that well. These coincidences are wonderful things - they bring us human contact and sociality without any effort. It's built into your life, no sign-up or registration required.
Maybe Facebook should be taking a page from the Caltrain.