Paul Shannon's book on Speedlink says this about the first air-braked wagonload services:
The launch of what was later to become Speedlink took place in early 1973 when British Rail ran its first air-braked wagonload train from Bristol to Glasgow, using its new long-wheelbase general freight rolling-stock such as the COV-AB van (later coded VAB for TOPS purposes) and the OPEN-AB open wagon (later coded OAA).
A second air-braked wagonload service, between March and Edinburgh, began in October 1973. Thereafter the network grew rapidly, with 24 services operating daily in 1976
Posts on this RMWeb thread date the Bristol - Glasgow service introduction to October 1972 (matching Wikipedia):
https://www.rmweb.co.uk/forums/topic/56920-br-air-brake-network/
David Larkin's book
Speedlink: Volume 2 mentions that it called at Bescot and Warrington.
Delving down a rabbit hole into the liveries, the COV-ABs (later VAB and VAA in TOPS) were the first production batch of BR-built air-braked wagons in the new air-braked wagon number series (starting at number 200000) built from 1969 onwards. The 6-digit number series for new air-braked vehicles lacked a prefix, unlike the B-series numbers. The original condition of the COV-ABs in Freight Brown livery is illustrated nicely by this photo of 200003 at Cricklewood open day in July 1969:
https://www.flickr.com/photos/blue-diesels/36966765982/ (Photo: John Senior - 53A Models of Hull Collection)
The freight brown livery has a bit of a red tinge to it and I've seen this referred to as "late bauxite." There's a bit more info in the introduction to Trevor Mann's book on vacuum braked wagons. The 1950 "Standard Colour Scheme for Wagon Stock" introduced "Bauxite Red." The later "Freight Brown" livery resulted from "a comprehensive review, associated with the launch of the new BR corporate image" that took place between 1963 and 1965. Freight Brown is described as "slightly darker, and more definitely brown, than Bauxite Red."
The colour of "Freight Brown" when freshly painted can also be gauged in this 1970 photo of 200088 in the bay at Stockport:
https://www.flickr.com/photos/blue-diesels/50713354898/ (Photo: David R Vickers - 53A Models of Hull Collection)
The yellow ABN symbol is missing in these photos, but this isn't surprising considering there was no air-braked network at the time! After the Bristol - Glasgow and March - Edinburgh trials, the "air-braked network" was launched in 1974 according to the information on this webpage (it also has illustrations showing how the ABN symbols looked on the wagons):
Air Braked Network, Speedlink and Enterprise wagon load services
The new maroon livery (with Railfreight symbol) introduced in 1974 did not utilise the ABN symbol - possibly it wasn't considered necessary after the introduction of TOPS codes on the wagons. I don't recall seeing any pictures of this maroon livery before, but that may well be because of the fading to brown mentioned in the previous post. This 1978 picture of 200215 on Paul Bartlett's site shows what the maroon colour looked like when freshly painted:
https://paulbartlett.zenfolio.com/brvaa/hc3c98422#hc3c98422
When compared side by side with the photos of freight brown livery, the difference is more obvious. I think this livery was the first appearance of the white "Railfreight" lettering. David Larkin has a picture of 200590 taken in July 1975 (page 13,
Speedlink Vol 1) where the wagon is only 4 months old and has the Railfreight lettering. Judging from the info in the two Larkin books on Speedlink wagons, the batch of VBA wagons starting from 200550, built from Dec 1974 onwards, may have been the first wagons painted maroon from new.
In a post on RMWeb, Paul Bartlett mentions that the red & grey railfreight livery was first seen at a
freight exhibition at Kensington Olympia in March 1979. This picture shows freshly painted VDA 210218, which would originally have been maroon:
https://paulbartlett.zenfolio.com/paulbartlettsrailwaywagonphotographs/h3ede5f39#h3ede5f39
Another picture shows VAB 200031 after repainting at Horwich in October 1979:
https://paulbartlett.zenfolio.com/brvaa/h257f581e#h257f581e
This picture from March 1980 shows an exhibition coach at York with Railfreight branding, which may have been associated with the Kensington exhibition. It has an open wagon (an OBA I think) in grey/red livery next to it, suggesting the train may have toured the country around this time:
https://www.flickr.com/photos/blue-diesels/9710406030/ (Photo: John Turner)
The notes in David Larkin's books say the OBAs were all delivered in maroon, with building completed in Sep 1979. In comparison, the SPAs were all delivered in flame red livery starting in June 1979. This flame red livery was a different version of the new grey & red colour scheme (minus the grey) which was intended for low-sided wagons. It was also featured at the Kensington exhibition, with 460002 seen here:
https://paulbartlett.zenfolio.com/brspa/h2cbf51f9#h2cbf51f9
With the build dates for the last OBAs overlapping with the introduction of the grey & red livery, it's possible that the last few OBAs may have been delivered in the new livery, although on Paul Bartletts's site both 110774 and 110785 are seen in maroon in 1979. Alternatively, that unidentified OBA seen at York in 1980 may have been specially repainted for the exhibition train. Looking at the Paul Bartlett site, there are 3 photos of VAB / VBB vans in traffic in the grey and red livery taken in 1980/1981, suggesting other wagons were being regularly repainted into the new livery by then.
A bit more info and some livery illustrations here:
British Rail TOPS era 1975-privatisation
