They have their own designated sick lines in C&W Depots. Freight maintenance is actually a very convoluted process. Freights are examined along their route at 'train examination' points. Unlike Passenger coaches, freights do not have much equipment below the shell. Other than the compressor and some pipes, the underside of a freight wagon is largely empty. So, a pit line is not really necessary. The sick lines in the C&W depots do have pitlines available, but again, they are not really required for regular maintenance.
The 'train examination' points and freight yards along the route of the freight train will maintain and repair the coaches based on requirement. For originating freights, the rake is inspected, and faulty...
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wagons are replaced at the terminal freight yard.
The maintenance costs are apportioned between the zones based on home zone of the wagon that is maintained or repaired and the route of the freight train from which the wagon had to be removed for maintenance.