One is a Rail Head Treatment Train (RHTT) that deals with poor adhesion / leaves on the line, formed of Class 66 + two container flats with blue tanks + Class 66. These are often running up and down all day. Here is this part of the diagram on Real Time Trains:
http://www.realtimetrains.co.uk/train/H41456/2018/12/06/advanced. This goes down to the Canonbury Curve, where it has 9 minutes to reverse. I presume it will stop running in the next few weeks as the leaf fall season finishes. Because of this train,
signalers often switch the 08:49 Moorgate to platform 2, which in turn leads them to divert the 08:52 Brighton to platform 4 and often delays all manner of other services...
The other is a GBRF Freight from Doncaster to Tonbridge with the headcode 6O75. Whenever I have seen this, the consist is formed of Class 66 + black painted open spoil / aggregate wagons (what would be termed 'gondola cars' in the US):
http://www.realtimetrains.co.uk/train/H45798/2018/12/05/advanced. This heads for the North London Line at Belle Isle (between Copenhagen Tunnel and Gasworks Tunnel), just before the point where the Thameslink services diverge for St. Pancras. This doesn't seem to run every day.