While only a mile per 8 hour shift (and one shift per day) doesn't seem terribly fast on the face of things, ordering such a vehicle for California would permit the electrification of the LOSSAN-South corridor within six months of starting work (assuming of course that the substations and autotransformers were built equally as quickly). The main obstacle, of course, is that without the planned tunnels through areas such as San Juan Capistrano, San Clemente, and Del Mar there is likely to be significant NIMBY opposition on aesthetic grounds (as well as some misguided environmentalists in those areas who believe that wooden or steel poles are more harmful for the environment than are diesel particulate emissions).
Two possible solutions arise that permit partial electrification of the corridor while the other enabling projects lurch forward; they may even be possible to do simultaneously:
1. The first, and less ideal solution, is to purchase dual-power locomotives such as NJT is doing (ideally, just changing the paint job and in-cab signaling system) for the Surfliner and Metrolink, both of which are planning on purchasing or rebuilding locomotives as it happens. Alternatively, a hybrid multiple unit such as Bombardier's Autorail Grande Capacité could be bought. The latter option is preferential as it can take advantage of funding for additional coaches for the Surfliner and multiple units have both a superior acceleration and public relations appeal compared to a normal locomotive and car consist (being as they would be new to Southern California and thus generate plenty of free advertisement through their novelty).
2. The more preferable option is to make this a mainly Metrolink project. The Orange County Line's additional service has its southern terminus in Laguna Niguel, ending before any aesthetic or environmentalist NIMBY concerns would arise. Being a stunningly unattractive corridor, the only real opposition I foresee is from those concerned about potential health hazards from overhead wires, a false concern which should be easily addressable, as well as potential takings for substations and transformers (however, those should fit within stations or in industrial parking lots fairly easily). If at all possible, the electrification should also be extended along the San Bernardino Line, fulfilling the initial promise of Metrolink two decades ago. At the same time, Metrolink ought to beg, whine, plead, and threaten to hold their breath until the FRA gives in and grants a waiver for the use of UIC compliant EMUs for their superior acceleration capabilities. This would provide a ten minute reduction in travel time to Los Angeles for both Laguna Niguel and San Bernardino as well as aid congestion reduction on LOSSAN.
Ignoring for a moment the cost of train sets to take advantage of the electrification (as rolling stock replacement is already scheduled), this ought to be a a fairly cheap endeavor with rest of world electrification costs in the realm of 1.5-2 million USD per track-mile. In terms of capital cost then, this should be on par with Metrolink's Rotem car purchase. Network Rail quotes a 33% reduction in maintenance costs for electric rolling stock (page 5) and a fifty percent reduction in fuel costs (plausible given the proximity of San Onofre nuclear power plant, however I suspect that the difference diesel and electricity prices is greater in Britain than in California). Combined with the greater patronage the new service might expect, especially with fares that can hold steady or even drop during times of gasoline price increases, it seems reasonable to posit that electrification would pay for itself over its replacement lifetime.