I see you are from Bedfordshire Theo,
as a long time absent Bedfordian and a long time 4mm and now 7mm DCC kit build fanatic who does`nt use any `stay-alive` gubbins
I would say you have possibly poor track laying and poor loco pickup issues. (both aggravated by unsprung/compensated loco chassis)
Poor pickups will cause arcing between pickups and wheels which will produce black gunge buildup.
So, select a length of track, clean it as you normally do. Select a loco, clean it, check all pickups are where they should be and are in constant contact with the wheel ( particularly if there is axle sideplay)
get yourself some Peco PL-64 Power Lube .....apply a TINY SPLIDGOEN of power lube to the backs of wheels/picups..... and see the massive improvement.....
and remember 90% of domestic dust is from human skin !! so when operating wrap up in a couple of bin bags !!!!
JOhn