I’m a big fan of the planned expansion of the MBTA’s Fairmount Line. Its a little used commuter rail line that runs from South Station to Readville. In between are some of the densest areas of the city that don’t have ready access to subway service. The MBTA has (slowly) been adapting the Fairmount Line by adding stations, in an effort to provide more service to these communities.
Some supporters of the expanded Fairmount Line have been pushing for DMUs, which are self-powered vehicles that could operate on commuter rail lines but might be able to provide more subway-like service.
When the MBTA looked into this option, they found that DMUs had significant benefits — quicker acceleration, lower operating costs and environmental benefits. The downsides are 1) money doesn’t grow on trees and 2) you’d be introducing a whole new rail car into the MBTA’s system which would bring a host of maintenance and training costs.
The study brings up one very interesting conclusion — DMUs would likely result in a about a six minute improvement in travel time. Half of that comes from its superior acceleration (against the big diesel trains) but the other half comes from the fact that they have more doors, and its easier to exit.