Philadelphia is by no means the perfect urban form, and it has a lot of political handicaps to overcome, some self-inflicted and some imposed from outside. But it has a lot going for it, and a lot of that includes legacy infrastructure from the Pennsylvania and Reading Railroads, and the PRT/PTC and Red Arrow Lines. This city would not be nearly as livable if we didn’t have the physical remnants of Alexander Cassatt’s and Merritt Taylor’s corporate empires scattered everywhere. In fact, that’s the only way, in the long term, to deal with a far-flung and slowly growing city of 1.5 million, in the heart of a 5.8 million person metropolitan region (and a 50+ million megalopolitan region). The 20th Century American dalliance with roads and the automobile is souring, and here we cannot accommodate the scale of volume that we are already attempting. Bicycles are a good last-mile solution, but don’t scale to the distances we often travel in our daily lives (a problem not as prevalent in more compact cities like Portland or Boston).
In short: we have trains, and thank goodness, because we’re going to need them.
Sic Transit Philadelphia 
Bicycles are also, for all their cheapness and low carbon footprint, insufficiently flexible to serve all the varied people in a city. They’re the vehicle of choice of the single, active, risk-taking twenty-something for a reason. But I don’t think I’ve ever seen the hippest Philadelphia twenty-something pack their infant on their bike along with their Apples to Apples game and bring them across town. Bicycle use doesn’t age well, and while cities should definitely try to be bicycle accessible, it doesn’t even begin to be a solution for the entire populace.
Besides, as fast and direct as bicycles are for those who use them, there’s no reason transit shouldn’t be faster. I certainly know New Yorkers who were poor enough (or thrifty enough) to save the $2.25 and bike the length of Manhattan; I’m pretty sure that this is only a logical choice when one is time-rich and money-poor. The fact that biking is time-competitive with transit in Philadelphia seems very strange to me. (Admittedly, Philadelphia also has significantly less street traffic, both of the automotive and the pedestrian varieties.)
That’s true as far as current conditions go; we don’t have much of a vehicular cycling culture here in the US (outside of Portland). Copenhagen and Amsterdam have much more diverse cycling populations, in part because their street design allows people to bike without taking their lives in their hands in heavy auto traffic. So you see more middle-aged cyclists there, and children riding behind in the bakfiets.
That’s fascinating. There’s still some amount of ableism associated with bicycling (which is not to say that those that can shouldn’t, just that we shouldn’t set everything up to rely too heavily on it), but I would definitely believe that lugging your kids around in some kind of bike-cart is lower-stress than trying to take them on the bus/subway.