Bulk coal freight is run in South Africa using Electric-Diesel mixed formations, complete with distributed power to make it really complicated - Locotrol II controls the electrics, the diesels in each group of locos are slaved to the electric using standard multiple unit working equipment.
This is done basically because the money is not there to go over to a full electric solution, and because it makes it fairly simple to take the train through a coal loader without attaching other locos to what generally operates as a semi-fixed formation.
... and you might see a diesel with a DIT (dead in tow/train) 'leccy behind it or vice versa.
A good method of shifting a loco from one place to another without the additional costs of sending a light engine under its own power - staffing, paying for a train path etc.
Different operating conditions will make the economics of this decision work differently - for example a vertically integrated operator will not need to worry about paying for a train path.