I don't think the 5-car Class 710/3 units destined for the North London Line have actually been built yet. I believe some of the 378s displaced off the Watford DC have made their way onto the NLL though.
Edit: Thread on RailForums says that several 5-cars have indeed been built, and delivered, and run as test trains (apologies for the misinformation).
are 5 car 710s running on the Watford DC still? Will these be made permanent?
What will happen to the class 378s which still run on the line? Surprised that there is a mix of stock used as I was under the impression the 710s were to take over the entire service.
The plan was that four car 710s will be on the Watford DC trains with an extra train every hour.
Five car trains will run other the other Overground lines.
I've seen 710/3 units running on the DC line at least until last week (didn't travel on Monday) and the service is currently curtailed at Kilburn High Road due to the flooding and so I have not noticed any 710/3 running so far this week.
With the currently commissioned 710/2 fleet (some units have not yet made it to London), there are insufficient 4 car dual-voltage 710 units to run all of the DC line (8 units needed) and GOBLIN (6 units needed) services including maintenance spares. Some of the 710/2 are also in service on the lines out of Liverpool Street, hence the use of the five-car 378s units on the DC line has to continue at least until all the class 710 are available.
Currently, it makes little difference whether the five-car 710/3 units run on the DC line (allowing the cascade of the remaining 378s) or on the North London Line. However, for the short term at least, running on the DC line would mean less staff training is needed as the type already runs on the route.
It will be interesting to see if any increase in demand for the DC line trains, due to the Bakerloo being thinned out, means that five car units are needed here anyway. It struck me as quite strange that TfL decided to replace a three trains per hour service of five car units with a four trains per hour service of four car units as this gave very little increase in capacity and the trains can get quite busy especially when there are problems on the mainline.