The trouble with this is that the trains have to have both a full set of diesel generation equipment for non-electrified operation and a full set of power conve

The trouble with this is that the trains have to have both a full set of diesel generation equipment for non-electrified operation and a full set of power conversion equipment for electrified mode, plus some way of switching between the two. The class 800 doesn't seem to have done a terribly great job of this; there's been huge teething problems with their introduction and rumour has it that the underfloor diesel engines simply can't operate at outside temperatures above 30 C or so due to inadequate cooling in order to fit them into the space available.