Hmm... A bit of conspiracy theory - Class 325's are very similar to Class 319s. Perhaps Northern drivers are learning the route with electric traction ahead of a new service?
If it were the case, I'd assume that timing/performance trials would be necessary long before driver training was required, making the former more likely at this point. Plus, while the traction equipment and design is similar, I don't know how alike the cabs of a 325 and 319 are, having never been in the cab of a 319 to compare.
Getting off topic and deep into speculative territory, if 100mph electric units proved that much nippier than the existing 75mph DMUs then extending the Metrocentre - Morpeth locals to Alnmouth would be a useful way to improve service frequency at Alnmouth. More likely, though is that, whatever the purpose of these runs, Alnmouth provides a handy electric turnback: A diesel engine fitted 319 operating Metrocentre - Morpeth wouldn't have an issue using the existing unelectrified turn back at Morpeth.
