Class 20 was intensively used in both freight and passenger service. Typical duties included international passenger trains from Brussels to Luxembourg with the trains continuing to Switzerland and Italy with French locomotives to Basel in Switzerland, Swiss locomotives in Switzerland and Italian locomotives to the final destination in Italy. Freight duties included heavy trains from the port of Antwerp to places like the yards at Stockkem, Montzen, Gent and Zeebrugge. Class 20 locomotives were seen with all types of SNCB hauled coaching stock over their time in service starting with Types K, M2 and M4. Later on they hauled double deck M5 and M6 coaches on peak hour trains, mostly between Brussels-Midi and Jemelle. Until Class 13 arrived they were the only SNCB locos that could pull 1100 ton freight trains on the difficult line from Luxembourg to Brussels. Class 20 was not equipped to run in multiple. Multiple working with the class was tried with a driver in each locomotive (a system of light signals was used to inform the driver of the second locomotive of what was going on) soon after introduction but the practice was quickly banned because the current demand of the pair caused problems for the electrical substations which were then in use.