I was wondering about the idea of charging off overheads from a train that only has third rail pickup.
I have an admiration for what Vivarail are trying to do, however I do feel they may have backed the wrong horse so to speak. Porterbrook are in the process of doing the same thing that Vivarail aimed todo with a few 319s. (slinging diesel generators underneath them) Vivarail have now moved it on with this approach of 'flexibility', but the testbed they are using is still an old underground unit that apparently requires significant work to bring it up to mainline standards. Add to that there is surely nothing stopping Porterbrook again doing the same thing with an off lease 319 once proof of concept with the diesel generators is done. It is great being an R&D company, but generally you need someone to pay you for that R&D rather than having a competitor just copy your ideas on a more marketable product.
That said, I don't think Marston Vale will be the only place Vivarail units turn up. The use of them has been mooted for the Borderlands line, particularly if it allowed an increase in frequency and extension around the Merseyrail Wirral line loop. Having not seen any Firm proposals about the next W&B franchise, and the devolution of tendering these services to Cardiff, plus the political complexities of cross border / England only services, it has been almost impossible to work out if this is a genuine possibility or just noise.