Bus transit is flexible; it can evolve as a community evolves. It's easy to open new bus stops and create new routes. That's good if you're interested in accommodating customers' preferences, not so good if you're interested in simply giving customers what you think they should prefer.
Enter rail transit. It may seem strange, but many planners like it precisely because it's inflexible. You can build it to serve areas with high employment concentrations, and since it offers a feeling of permanence, the hope is that future growth will cluster around the line.
But there is, of course, no guarantee that growth with cluster around rail lines. There's not even any guarantee that rail lines will prevent existing development from leaving.
- One example of the way the [Washington, D.C.] region has grown was what happened after WorldCom took over MCI. The bulk of MCI employees worked in the company's Washington headquarters and an office in Pentagon City, both within short walks of Metro stations. They eventually were shifted to WorldCom's campus in Loudoun County, more than 15 miles from the nearest subway stop and nowhere near a bus line.