I know I'm getting old but in the 1950s and 1960s a 10 car train was fairly common.
I have a copy of a RCTS survey diligently taken for the Sheffield area on August 11th 1962. Those bewailing shortcomings on XC today would love it.
11s were plentiful, St Pancras - Edinburgh, Bristol- York, St Pancras - Glasgow, Bristol - Bradford to highlight just a few.
12 from Bristol to Newcastle, Newton Abbot to Bradford, Exeter to Bradford.
13 from Paignton to Bradford, Newquay to Newcastle.
I wouldn't be surprised to hear of 14 through York on the ECML.
There were still long trains being split in Sheffield for Bradford and Newcastle about 1980.
Through corridors back then, all rolling stock compatible too!