I was a coal controller at Nottingham and Doncaster.
Dr Hoo is correct, we weren't allowed to send "canopy" wagons as they were called to certain pits.
A number of HAA's were fitted with side canopies to stop coal dust from being blown off in transit, they were recoded HBA, HCA, HDA and HFA, they had various permitted maximum speeds.
Denby colliery north of Derby was canopies only, residents adjacent to the line had complained of coal dust being blown onto washing hung on washing lines so we had to make sure we always had a 36 set of HFAs spare. Millerhill to English power stations was another where local residents had complained about coal dust being blown off trains crossing the Royal Border Bridge at Berwick.
Some collieries and disposal points would accept anything, Bentinck, Avonmouth, Oxcroft, Rugeley come to mind.