Michael Bell wrote:
This may not be the right forum for this question, but what the
hell!

Standing at Hitchin station and wistfully watching the fast trains
going through, the wheel-on-rail noise of class 91 + Mark IV
coaches seemed much louder than the Eurostars. (Comparisons with
HSTs are difficult because of their noisy diesel engines - at full
chat as they pick up speed going North). This was just a casual
observation. Is this generally accepted? Are the reasons known?

It may be because Eurostars (Class 373) are articulated, with one bogie
shared between two carriages except at the ends of each unit. Thus at
any instant while one is passing you, there are fewer bogies in contact
with the rails within x metres of your position than with a rake of Mark
IVs.
--
Richard J.
(to e-mail me, swap uk and yon in address)