From my Co-Bo World website....
Paul Harrison writes,
I have researched the history of the ICI hopper wagons including a short section on the Class 28s; Trials using a pair of Metro-Vick Co-Bos (later class 28) were conducted in May 1963 and they demonstrated superior braking power to the EE Class 40s which were also being assessed as replacement for the 8F steam locomotives. These trials on the 21st - 23rd May used D5711/D5714 in multiple hauling 19 wagons on the third day.
The Class 40, thought to be D228 had suffered damage to the wheel tyres and brake linings and hauled a maximum of 16 wagons. Tests with 19 wagons were scheduled for the class 40s but were cancelled due to the damage sustained. Further trials took place in June when D5700 which hauled a 19 wagon test train unaided for four days in a row.
Full scale introduction of these locomotives never took place, the final trials taking place in March 1964. The main problem was maintaining sufficient brake-force on the descent to Cheadle Heath despite the Co-Bos having good haulage abilities. The plan was to transfer 8 of the class to Trafford Park shed on a permanent basis.