These units have special air intakes that collect air from the bottom of tunnels to avoid ingesting their own superheated exhaust gasses from the roof areas of the bore. Diesel Tunnel Motors pull their fresh air in from vents near the lower rear of the hood. To expand further the problem was that when the cooling air was drawn from the top of the bore it was quite hot and did not cool the radiators adequately, causing the engines to overheat and shut down. The problem was with cooling air not combustion air. If you will notice the radiators get larger as the unit power increases.
Engine cooling is one factor limiting power output.
They are distinctive because they can be spotted by the see through grills located at deck level in their rather squared off rear bodies. Tunnel motors are also distinguished by the lack of the EMD round radiator fans on the top of the unit. I'm always looking for more tunnel motor photos, and if you have some that you would like to have posted here, e-mail me, click on button below. I'm looking for any tunnel motor repaints in other roads. The Union pacific is selling off some of the SDTs and they will be in a rainbow of colors soon, so keep your eyes open and a camera by your side.