Busses are cheaper than trains!  (see one example at Bus vs. Train)

Of course to anyone with more than 10 minutes of Econ 101, this is obvious.  Trains are inflexible and have huge fixed costs.  Busses are flexible and can be deployed in weeks not years. 

So why do politicians and bureaucrats continue to push for rail "solutions" to transit issues?  I suggest that it is because rail systems fit the politicians and bureaucrats view of social planning:

• Rail systems support the development of urban concentration around fixed stations and dense urban development is the goal of most social planners. 
• Rail systems are huge public works projects and politicians like to get credit for "doing big things" - even if those big things are white elephants. 
• Rail systems have huge fixed costs that require raising big chunks of tax dollars with sales or use taxes...taxes that don't expire with the project, but typically live on to be reallocated to the next pet project. 
• Finally, rail systems are expensive and for the most case, built by union members who are reliable voters for the politicians that vote for rail systems.