The Big Picture

With the delay of a federal grant long planned for Caltrain modernization, there is fear and uncertainty on the peninsula rail corridor. Everything about the project is being put back into question by voices on all sides of the issue. In times like these, it helps to step back and look at the big picture. The big picture has not changed since 2008, and there is a logical flow to it that remains true regardless of the funding situation.

A requirement flow diagram shows a hierarchy of requirements, things that are needed or wanted, and how they relate to each other. The way to read it is to follow along the connectors between boxes. When reading downward, the next box down answers HOW the previous box is to be achieved. When reading upward, the next box up answers WHY the next box is necessary. A simple and intuitive example is provided at right.

Next, we move on to a more complicated diagram that represents the blended system in general, including the Caltrain modernization project. The derived attributes at the ends of the requirement tree are highlighted in green.  If you delete any of the green boxes, all of the boxes that depend on it above are negatively affected.

For example, if you delete level boarding, then you can't reduce station dwell times, which means you can't increase Caltrain average speeds enough to allow operating peak hour traffic, which in turn means the blended system won't work well, and HSR may need to build four tracks all the way.

For another example, if you delete train doors that work at the same height as HSR, then Caltrain can't share platforms with HSR, which means bigger stations and limited capacity at SF Transbay, so Caltrain won't be able to run all trains into the downtown core, which in turn will hurt Caltrain ridership and increase congestion on highway 101 and I-280.

(Click to expand to full size)

This is a useful way to think about the problem, and reveals three important ideas: (1) the mere lack of funding won't make the problem change or go away, (2) the technical approach pursued by Caltrain is sound, if only partially effective, and (3) hacking away the entire HSR side of the diagram doesn't fundamentally change the solution ultimately needed for Caltrain modernization.