Only the SD40-2 and SD45-2 were produced in "tunnel motor" variations, the SD40T-2 and SD45T-2.
SD40-2 and SD45-2 are both 68' 10" over the coupler pulling faces.
SD40T-2 and SD45T-2 are 70' 8".
SD40-2 and SD45-2 have radiator fans on the roof that suck air through flush radiator grills at the top of the hood.
SD40T-2 and SD45T-2 have internal radiator fans that bring air in through a much larger intake that's at the level of the walkway, and blow it through radiators mounted on the roof.
The extra frame length is to accomodate the redesigned radiator section; from the engine compartment to the front, they are the same as an SD40-2 or SD45-2.
The SD40T-2 and SD45T-2 were only ordered by the SP/SSW and DRGW, so they have some unique details typically specified by those roads (lights, jacking pads, plows etc). The "T" designation is for "Tunnel"--the different cooling system eliminated cooling problems in tunnels experienced with regular EMDs. The idea is that the tunnel motors draw in the cooler air lower down in the tunnel, since the air at the top of the tunnel is very hot from the exhaust.