In the case of A class loco's, it's real easy...you can feel the ground
shaking from the vibration in them, and their turbo whining is much
noisier than the N's (although CLP's were nearly as bad). I reckon the
567's tend to have a smoother engine note at low revs, the 645's have a
'lumpy' beat (lower idle speed?).
After much practice, I can usually stand at the bottom of the ramp of
Spencer St plat 7 and tell which class of loco is on the WCR train -
that's only distinguishing a 567BC (B class) from a 567C (S class), but
it takes practice!
Cheers,
Craig.
-- Craig Haber albatross@harnessnet.com.au Manufacturing Systems Engineer (almost) Web Page Designer, Harness Racing, Railways, and Essendon Football Club fanatic http://www.harnessnet.com.au/