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.<p>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.