You are correct re the gold lines... I believe the fastest official speed for a Duchess was 114 mph for one of these on a full test load (a world record at the time!) ... my understanding is you can't get the coal in quick enough to get the max output out the boiler...
TE of an A4 being approx 34,000lb
TE of a Duchess being 40,000lb
The Princess Class though has the higher TE at 40,285 along with the King at about the same!
Typically the A4 was built for high speed light loads of say 220T and long range, whereas the Duchess was built for speed and brute power many believe, many trains being 400T plus...
The debate over which is aethetically the best is in the eye of the beholder... as for comparing performance of any of the express passenger types its like comparing apples with pears.... they were all designed with different objectives giving us a wonderful variety of express power over the decades.
Regards ICLOK
Aaaaaaaaaaaaaaaaaaagh Creeper!!!!!