Henry Littleton writes:
A few weeks ago, I did a day return trip on the flagship Amtrak 'Acela
Express' down the Northeast corridor between New York Penn station and
Washington. The journey took 2h45m at 150mph...

The top speed permitted on that trip is 135 mph, and that's only on a
short section. The Acela Express does run at 150 mph on a short section
of the New York to Boston run, but 125 mph is a more typical speed. The
higher top speeds mostly give publicity value, until and unless a good
deal more of the route is further upgraded to support them.

(Can you say "Intercity 225"?)

Now, compare this with Virgin West Coast from here in Euston to
Manchester. The journey time is very similar at 2h40m but obviously
the distance covered is far less than above.

Well, it's about 18% less.

* New York Penn station to Washington - 225 miles - 82 mph average
* Manchester Piccadilly to London Euston - 184 miles - 69 mph average

(The average speeds shown are based on the times given by Henry;
I haven't done a detailed search of both routes' timetables to check
the exact fastest or typical times, but a quick look shows that his
numbers are about right.)
--
Mark Brader, Toronto Rocket, 1829: The first 30 mph train.
TGV-A, 1989: The first 300 mph train.

My text in this article is in the public domain.