Ozzie,
I am struggling for an answer. It does not make sense to me either.
The difference between todays track and that of 50 years ago may be something like this.
In the good old British Railways days of steam, the locos had very little springing. The track used to flex under the weight of the locos which were relatively rigid with only very little springing. Safe ride.
Now the track is very much stronger, rigid in fact, and the diesel and electric locos and stock are sprung more. Safe ride.
So you put a rigid loco on a rigid track: with no springing at all you ought to expect a rough ride. Hence you have to go more slowly for safety and avoid bouncing right off. Unsafe ride.
What has that got to do with 9Fs? I am not sure. Can anyone take the reasoning further?
Could it be that EU Regulations, to whom we must all do obeisance, make it illegal to have a flangeless wheel in case the the thing derails? Is it something to do with unsupported length at the crossings at high speed points?
Any body else know?
Colombo