The other aspect I find fascinating is that you're actually going down pretty steeply on both sides of the bay when you're in the tube. When you start on the Oakland side you’re above ground, say 50 feet, and then you you plunge under the bay, 130 feet below the surface. Then you have to come up on the San Francisco side to connect to the tunnel at Embarcadero — though you're still about 100 below ground level at that point.

The experience as a passenger is that passage is pretty flat. You kind of feel like you're just going straight across. But there are a couple places where you change elevation pretty significantly under the water.

In its nearly 50 years of existence, why has BART taken so long to extend into Santa Clara County? And only normally protruded into San Mateo County and never expanded into any of the North Bay counties? Submitted by Mark

DB: The simple answer is that most Bay Area counties had the choice of opting into BART in the early '60s, and some opted out, including Santa Clara and San Mateo.

There was some talk about going up to the North Bay, specifically Marin. There were engineering studies that showed it was feasible to go to put track below the roadway on the Golden Gate Bridge. Three engineering studies found that was fine, and one sponsored by the Golden Gate Bridge District did not support that idea. However, when San Mateo County opted out, the backers of BART no longer thought that the Marin idea would be feasible. On top of that, the Golden Gate Bridge District never really liked the idea because back then BART would have been competition for the service they were offering.

So BART has only gone into Santa Clara and San Mateo in relatively recent history — the 1990s in San Mateo County's case, and soon in Santa Clara County, where the first extension there, to San Jose's Berryessa neighborhood, is expected to finally open late this spring. That's because people in those counties reconsidered and passed taxes that allowed BART to expand there.

Why are there news kiosks in the BART station that are closed? Why doesn't BART let people run these news kiosks and what are their history? Submitted by Brad Meyer

DB: Those were created for the San Francisco newspapers. It was really just the Chronicle and Examiner. It wasn't like an old-fashioned newsstand like you might see outside of a subway station in New York or Chicago (or on the platforms of some stations in those cities). But guess what? Newspaper street sales, which used to be a big thing, have dried up because of this digital medium we're using right now. Street vendors and newspaper boxes are a thing of the past. I think that the final nail in the coffin for the kiosks was the introduction of smartphones, because now people are carrying the news with them. Some of the kiosks are still there. but many have been removed.

There is a pretty good story connected to the old kiosks, though, where somebody discovered that at a couple of the stations (Montgomery Street is one of them) that there is a very odd acoustic effect. If you go to the place where the kiosk used to be and talk in a normal voice, there is an echo effect. And it was enough of a thing that I know that some local reporters went down there to demonstrate it.

At the West Oakland BART Station there are ads right next to the third rail. How on earth do they change those out without getting electrocuted? Submitted by Jay Quigley

DB: The third rail is something you don't want to touch under any circumstances. It's very high voltage and coming into contact with it could mean instant death, or at least extremely serious injury.

BART is shut down for a part of every day. Trains stop running around 1 a.m. and then there is a window of time between 1 and 4 a.m. when a lot of maintenance takes place. So in the case of working on these ads, BART does that during that overnight closure and shuts down the third rail.

I was told by Jim Allison at BART that they call it “laying down a blanket,” as if you're covering the rail with a blanket to make it safe. They're not physically doing that, of course. Once the rail is de-energized, then the crews can work safely there.

Why does it seem like the escalators are constantly broken? Submitted by Dan

DB: There are a variety of reasons that escalators fail. Some are exposed to weather. Some are exposed to abuse of various kinds. There were some escalators that were being used, essentially, as restrooms. Lots of garbage winds up dropped on certain escalators at some stations. The other thing is that they get a lot of use, lots of wear and tear.

But many of the escalators at this point are also very old. That makes them harder to maintain and also creates a problem few people were thinking about when BART was new. In one case, the original manufacturer of many of the escalators in San Francisco's downtown stations, and some others, has gone out of business. To do simple maintenance, BART has to go out and hunt for spare parts. And if things break down altogether and they have a serious mechanical failure, they have to find somebody to make some of these major parts.

At San Francisco's 16th Street/Mission Station a few years ago, which is one of the places that had these old escalators made by a defunct company, they put up a sign: “This unit will be out of service until ...” and then there was a blank space. It was out of service so long that somebody came along and wrote “the rapture" in that blank space.

I started to track how long it was taking to get that escalator up and running, and it was out of service more than six months. It turned out that BART actually had to find somebody to make a part called a bull gear pillar block.

Looking ahead, a lot of these escalators are getting completely rebuilt. They're not just getting overhauled — brand new escalators are being installed. It's an expensive project and will be going on for years. BART just announced that it's going to build protective canopies over all the escalators that have been open to the sky, so that they'll at least be protected from the weather if not from other forms of abuse.

Why does BART announce elevator status all the time? Submitted by Eric

DB: There is a large part of BART’s ridership that depends on elevators to get through the stations.

On one level, it's just a courtesy announcement, but on another level, it's sort of a crucial announcement for people who have no other way of accessing a BART station than using these elevators.