I am pretty sure OPB is referring to the X2000 rolling
stock, which uses active tilting, instead of passive
tilting, like the Talgos. I read a brochure the X2000 folks
gave out back in 1993, when an X2000 set was touring
this country, that said that the X2000 could go around
curves 30-40% faster than conventional rolling stock.
This was because the X2000 equipment had steerable
trucks, This cut travel time between Stockholm and Goteborg
from 4-1/2 hours to 3 hours -- all without running in the
true "high-speed" range (+150 mph)!. VERY impressive,
especially considering that the X2000 equipment back
then maxed out at 156 mph. The brochure said the tilting
was only for passenger comfort, and had nothing at all
to do with enabling faster speeds around curves.

The smoothest ride I ever remember on a train was a ride on
Amtrak's "San Francisco Zephyr" back in 1983 through Wyoming
on the UP's then smooth-as-glass main line. This was on a
Superliner, too, at 95 mph!

My only beef with the Comet cars is that their doors cannot be
trainlined. They have traps, and thus must be opened by a crewperson.
This will greatly increase station dwell time, as there will be no way
any passenger on the platform can know ahead of time which door(s)
will be open, and even on-board passengers will need extra time to
get to the only door(s) that will open. Not practical at all, but at least
these cars will only be used on two San Joaquin trains.

Margaret