It appears to me that some of the posters on this thread are not fully aware of the history and use of RDCs or their more recent cousins, the DMU. RDC's were marketed as replacements for some branchline services but their largest use was in commuter service (see Boston & Maine, Pennsylvania-Reading and others). DMUs, as built and marketed by Colorado Railcar, are fully FRA compliant, use proven diesel technology and are designed to operate either in multiple-unit or single. This means flexibility to meet ridership changes. Both DMUs and loco-hauled trains represent variations on the heavy-rail concept. Even the most optomistic of SMARTs ridership projections would not justify the expense of electric overhead. DMUs remain the most cost effective option and use "off the shelf' technology.

By the way, if anyone remembers the SPV-2000 offered by Budd a while back. That technology failed due, in part, to high initial cost and low reliability. The insistence that they be able to double as loco-hauled coaches created a transmission nightmare.