For their time, they certainly were very advanced: (don't know about the UK, but looking elsewhere)
– Some of the Canadian ones went through the "National Research Centre" wind tunnel for streamlining and flow analysis (the models still exist)
– Oils and superheating – oils had to be designed so as to not burn in the high temperatures associated with superheating;
– materials work; light/strong alloys for boiler courses, stay bolts, etc;
– Aluminium – some superstructure (at least "over here"
were aluminium;
– the fully cast beds, cylinders, etc. are incredible to see, and pushed casting technology;
– welding – look at the German Kriegslok system for the movement from rivets to welded technologies; the west followed (or went in parallel – don't know)
– time keeping – time keeping, and just organization of complex interactions needed advancing as forced by the railways;
– non destructive testing methodology most likely had been advanced by the railways;
– computer controlled trains, computer controlled signalling, etc – even the knowledge advancement of the old signal frames in the UK might have been profound for the time.
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Yes, certainly, even a lowly olde-school Boeing 747 is a complex beast, and nothing like an A380 or 787 in complexity, but making an A380 at the height of development of the steam era would have been impossible.
For those doubters; is the era of advancement of air transport past its' prime? With the demise of the Concorde, what really has changed in aircraft design in the last 20 years? Fly by wire, sure, but it's old technology now. Carbon-fibre? sure, but its old technology now. Radar?? sure but…