Yes Jonathan is right.
Traction motor blowers, exhausters, compressors, air reservoirs and other general ancilliary equipment. Class 37s did have connecting corridor doors (hence their distinctive split headcode windows) though I believe they were never used operationally.
Early diesel locos had the "bonnet" design following the LMS prototypes numbers 10000 and 10001. The Chief Mechanical engineer of the LMS at the time (R C Bond) insisted on the design as he wished to afford the crew a measure of protection. Later designs did not follow the principle.