A large part of the developers I know (ie met physically) are not even aware of what JSON is (although it definitely contrasts with the web/ruby/python/javascript world).
Another point is that API to parse XML are built-in (and widely known) in almost all platforms, whereas JSON requires external libraries on some (I believe .Net, Java for instance).
I think that's exactly it - XML integration, serialization to XML tends to be built-in to many existing environments. If the BART site is any clue, they are using ASPX/.NET.
The XML they are using is also very straightforward, elements-only, no attributes, namespaces, SOAP, other weirdness. It doesn't look any harder or easier to produce and consume than JSON. For part of their previous offering they are using a Google-designed format called GTFS which seems to be essentially well-defined CSV.
Encoding JSON is very easy. If that's all you are doing (e.g. only sending data to the client through say a web API), and you don't have a native library: build one.
Edit: also, I am a big fan of providing both. It can be done simply and cleanly, so why not.
Namespaces are a good idea. Having both attributes and child elements is a good idea. The combination of those two allows for easy, partial, versioning of your schema. The ability to impose an ordering is also a nice idea.
I'm not convinced. JSON objects have attributes and can contain child elements. Ordering can be provided by arrays.
Namespaces are also easy to create within objects; an example would be the "type" attribute from the Freebase API.
For versioning, many APIs are accessed through a different subdomain or URL (e.g. http://api.example.com/v2/). If you're feeling somewhat enterprisey, you can also wrap your response in an "envelope" object containing all the necessary details to implement your own custom protocol.
JSON objects have attributes and can contain child elements
JSON objects only have members. These are indistinguishable from each other and there isn't a separate species of them that is obviously metadata for the object itself. Of course you can use some special naming scheme to indicate those, but if everyone whips up their own naming scheme, consuming JSON from multiple sources becomes a maintenance nightmare, especially in the absence of something like an XSD.
Namespaces are also easy to create within objects;
The same arguments holds here: if everyone whips up their own version of namespacing, by using some special member to indicate the namespace, interpreting JSON from different sources becomes that much more of a maintenance hell. Moreover, many users will not use namespaces, as it isn't required. XML forces you to think about the structure of your data representation.
For versioning, many APIs are accessed through a different subdomain or URL
The physical URL through which an API is accessed should be entirely different from the virtual URI's used for namespaces and versioning.
Now I fully agree that XML and the various specs surrounding it are bloated and overengineered, but there are some sound ideas in there that a replacement should attempt to keep. JSON can be used for simple cases, but as soon as the problem gets slightly more complex, you start adding non-standardized extensions to JSON, to obtain goals that XML has foreseen. I don't believe they can replace each other: they both have their own niche.
The irony about BART is I once randomly met the guy who does their software (.Net). He doesn't even use BART; he works from his office in Portland, OR!
Probably most of the credit goes to the people involved in the project and their ability to champion their cause and provide these services, straightforward documentation that looks like it's written for humans by humans, etc.
Another part of it might be that BART has been highly automated and computerized since its design and inception so the data they offer has been available in some machine format or another for a long time. I remember there were a couple Macs with atrocious screen burn-in displaying all sorts of real-time info from the inside of the Berkeley Shattuck Ave Station Agent's fish-bowl in the early 90s - if you could hook it up to a Mac then, it seems natural you can offer it over the Web today.
Massive budget deficits, gross mis-management, fraud, MURDER, etc have all come together to ruin BART's reputation and make them an embarrassment to California government. This is just another attempt at fixing their ruined image until the next scandal.
I'm assuming that there's a set of functions for shooting innocent but rowdy civilians in the back, while they're handcuffed and on the ground face-down?
