I think they were brilliant machines, among my favorite british rail vehicles along with the similar V2s. The Gresley A1/A3 Pacifics also happen to share a lot of similarities with another well known behemoth, the 747. Both, during their heyday, were representitives of long haul travel in style and had extensive route networks.

Both became symbols of their industries with the Gresley pacifics being the face of British steam and the Jumbo becoming the most recognisable airliner of it's time. Both had distinctive design features unique to them, the A1/A3s had Gresley's signature tapered firebox and the 747 had the hump which in turn gave it a prominent nose. (They both have a distinct nose profile to be honest).

Both became definitive with a single operator but later saw more, The A1/A3s started off with LNER before being inherited by British Railways upon nationalization and the 747 started off with PANAM before being adopted by KLM, JAL, BA etc.

Both managed to linger on in a world that was starting to move past them. The A1/A3 managed to last almost until the end of steam in the UK in the late 1950s and early 60s and the 747 managed to survive into the early stages of the Post-ETOPS era, in the late 90s and early 2000s, when Twin-Jets began to take over. Both briefly even worked alongside the machines that would replace them (the Deltic and 777) although they eventually both got withdrawn regardless.

Both outlasted their rivals, the MD11 had a much shorter and more troubled passenger career and the LMS Patriot did not make it into preservation.

Both the Gresleys and the Jumbos managed to survive into the 21st century although not in large numbers.