It is true that in the US, we use generally use locomotive hauled trains for even 2 or 3 car commuter runs, but the rest of the world generally uses multiple unit cars for the same sort of serivce. For many years, the US also used MUs for lower density operations, in fact, Marin County was once served by NWP EMUs.

Here is a Powerpoint that was presented to the SMART board describing the Desiro proposal.

[www.sonomamarintrain.org]

The HP of a 2 car Desiro is a lot lower than the HP of a 4000 HP locomotive that would be pulling a similar number of people. I'm not a rocket scientist, but all things being equal, it should take less fuel to produce the power to run a DMU than a locomotive pulling a 2 car train.

It looks like Siemens believes that it can bulk up the basic Desiro to meet FRA requirements.