Class 70 emissions
 

I was standing at Highbury this afternoon and a freight train went through
hauled by a class 70. The billowing blue-gray exhaust gassed half the platform.
I've never seen this sort of smog with the more common 66s. Is this normal for
a 70 or was its emissions equipment probably broken? If the former how come an
HGV can fit some heavy duty particulate emissions equipment under the cab yet
a 70 foot long locomotive can't manage it?