Complex sentence: The wind-tunnel-tested, aerodynamic body and high power allowed the class to reach speeds of over, although in everyday service it was relatively uncommon for any steam-hauled service in the UK to reach even 90 mph, much less 100.
Split into simpler sentences: The wind-tunnel-tested, aerodynamic body and high power allowed the class to reach speeds of over, although in everyday service it rarely attained this speed. No regular steam-hauled rail service in the UK reached even 90 mph, much less 100.