The EMD's with winterization hatches on them (mostly older stuff, without modern cooling systems) use a hatch, with a door that can be opened, or closed. When temperatures approach, or go below freezing, the doors are opened on the hatches, and the air normally blown through the second cooling fan (the front one) is diverted, through the hatch, and into the carbody of the loco. This is an attempt to keep underhood temps above freezing, which can, and does occur, when an engine is left to idle. As the cooling system cycles itself, there are times the radiators might not see sufficient waterflow, to prevent freezing of the radiator cores. The recirculation of warmed air keeps the area under the hood warm enough, to prevent this. If you have a really cold area, you might even see both fans covered, with a dual winterization hatch.
On switchers, you might see a roll of canvas dropped across the front radiator screen, to slow the flow of air, through the radiator, due to constantly running cooling fan. These locos are especially prone to freeze damage, in the radiators, at idle. Same thing the truckers do, trying to keep the radiators fluid.
Not suprisingly, those same winterization hatches can, and will, slow airflow in the warmer weather, and if you are running them in heavy grade territory, with heavy tonnage, you will find they tend to overheat, more readily than the locos without the obstructed fans. Even with the doors open, the size of the opening in the hatch, is greatly reduced compared to the surface area of the fan opening at the shroud. Regards