I speak with some experience of Speedlink. Most trunk trains had a reservation system for the most important traffics , agreed for timetable periods , so that the yard supervisor at say Mossend would know how to plan the priorities for say 6Vxx to STJ , giving priority to "reserved" or priority confirmed traffics if there was a surplus for that particular train. It usually worked very well , and the liasion between the ground staff and the Speedlink / Regional controls worked very well. In a "one" railway , unburdened by track access rights - specials could and would be organized as needed , crews and traction provided. The overnight logs frequently gave examples of good proactive work in the small hours - where all sorts of things were done to keep traffic moving.
The levels of pre-reserverved or "Mandated" traffic flows were agreed between the Speedlink Service Group Managers in Crewe and York and the individual business sector managers. The mandates dealt with both length and weight, since both could be critical on the Speedlink trains. The Speedlink Performance Assistants at Crewe and York would keep a close eye on all the trips into the major yards to see what traffic had been released, and would then confirm the priorities with the relevant AYM. It wasn't left up to the AYMs to do their own thing, as they would naturally try to make life easier for their staff by doing the minimum number of shunts, so if (say) 18 wagons of scrap metal from Corby to Mossend ended up at Bescot instead of just 8, then the load might need to be split over three different services - requiring the yard staff to do a lot more work. If left to their own devices they would put the whole lot on the first available service, bumping other mandated traffic to a later train, potentially upsetting the customer because of late arrivals.
The northbound loads over Shap and Beattock were critical for the WCML services. From memory (and feel free to correct me if I'm mistaken) the trailing loads were:
Class 81 - 86 610 tonnes, but could be authorised for 650 tonnes.
Class 87 650 tonnes, but could be authorised for 710 (?) tonnes
Any tandem pair 1100 tonnes
Pair of 86s or 87s in multi 1100 tonnes, but 1300 tonnes if authorised and wired out by RCO Crewe (which was a daily occurence).
I do recall one night when there had been a strike the previous night, and there was a huge backlog of traffic. I managed to get every northbound train that night to run as two separate trains, to increase the overall trailing loads and lengths available. Again from memory that was:
6S73 Dover - Mossend (from Warrington)
6S74 East Usk - Mossend (from Warrington)
6S80 Warrington - Mossend
6S82 Bescot - Mossend
6S97 Gloucester - Mossend
A fantastic example of co-operation from the Regional Control Offices and yard staff to keep the traffic moving.