They work some kind of Crossrail spoil train to a disposal point in southern land somewhere. I think it is a GBRF job.
The train you are referring to ran from daily from Willesden to Northfleet,
but it ceased operation a month or so ago, with all CrossRail spoil from
Willesden now going to Calvert instead, although there are still trains
from Westbourne Park (Paddington) to Northfleet, although those are
strictly 66-hauled.
The class 73s which used to work the Willesden spoil train now operate
a semi-regular Hoo Junction to Cricklewood engineers train, which runs
up to four mornings a week.
MARK