An employee of the VBZ public transport authority discovered the body of a man in the tunnel shortly before 7pm, city police reported.
The initial findings suggest the man was struck by a streetcar but it is not clear how he wound up in the tunnel.
According to an autopsy, a tram crushed the victim in the late afternoon.
Shortly after 4.30pm, a VBZ driver piloting a streetcar through the tunnel signalled by radio that he had heard a loud noise.
Transport authorities initially believed the noise was related to construction work going on in the area.
Police are seeking witnesses who may have seen the man enter the tunnel.
The victim was wearing a black jacket and dark blue trousers.
Elsewhere in Zurich, a 12-year-old girl was rushed to hospital on Monday night for treatment of injures after a streetcar struck her as she walked over a pedestrian crossing on Badenerstrasse.
The incident occurred around 6.30pm.
Police believe the girl’s injuries are not life threatening.