It should be a simple feat in San Francisco, which has only two subway lines. But checking the acrossair website shows information only for the London and New York versions, and even those are said to be “launching as soon as Apple approves it!”

Update a day later: Speaking of BART, Streetsblog draws attention to the transit system’s “data transparency,” meaning that it allows anyone to use live data from its train control system to build applications, and lists the resulting applications on its website.