Although it didn't (quite) reach MCV, the Heaton-Red Bank empties used to be a rather hefty train - 20+ vans on occasion, and double-headed by a wide variety of odd pairings in steam days. In my spotting days, 46026 "LADY" seemed to be a regular performer on this train.
I believe the Heaton-Red Bank empties you mention were the return workings of newspaper trains from platform 11, which used to run to Newcastle and the North East the night before.
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While not exactly answering Paul's question about the number of vans in the make-up of the longest newspaper train, Martin Bairstow in his book "The Leeds, Huddersfield & Manchester Railway - The Standedge Line" [pub.Allanwood Press Ltd, Pudsey], gives the following information re eastbound newspaper trains over Standedge:-
From Manchester Victoria:-
12.35am to Newcastle, running non-stop Huddersfield to York via Leeds;
12.45am to York;
2.15am to Leeds, which shunted over to the "up" platform at Dewsbury to facilitate unloading.
Apparently, the return Heaton -Red Bank empties also collected these vans from York and Leeds, resulting in a very long train, leaving Leeds at 11.12am, and travelling via Dewsbury & Hebden Bridge to reach Red Bank carriage sidings.
These workings were of long-standing, and still current when the book was written in 1984.
I suppose platform 11 at Manchester was ideal for loading these trains not only for its length, but because there was also road access directly on to the platform.