[Just thinking (OP)]

Have you ever wondered why some steam locomotives have smoke deflectors and some don't. This may be an OHAS issue but why and for who? How do the smoke deflectors on a steam locomotive work and what are they for? You may read many stories of how they work and what they are for but you will never find the truth as the railways have left this as a mystery all the way to now. So if you don't know the answer to this question just ask why. 

[Halc]

They're there to push the smoke higher to prevent it from either blocking the view forward or to give passengers cleaner air to breathe.

How they work is pretty obvious, but it only works at higher speeds.  The bow wave coming off the front hits the deflectors that extent forward.  The air has to go mostly upward, the only direction not constricted. This pushes the smoke up further.

Smoke already has significant upward draft since they blow exhaust steam up the stack to give it far more velocity than it would have from just a fire burning with gravity to feed the air current. This expels the smoke vertically (making the deflectors optional) and has the primary benefit of putting a vacuum on the exhaust side which draws air into the firebox making it hotter and more efficient.

The deflectors also add some drag, which is insignificant for a train, but may contribute to the choice not to always include them.

I have stood on/near the tracks immediately after the passing of a long fast (100 km/hr) freight, and the air very much moves with the train leaving little resistance for any one car. Hence the streamlining of long trains has little effect except for making it pretty. They used to streamline the late steam models and the early diesel F units, but nobody bothers anymore it seems except for the really high speed trains.

[evan_au]

I didn't know what smoke deflectors were, so I had to look it up:

They are sometimes called "blinkers" in the UK because of their strong resemblance to the blinkers used on horses, and "elephant ears" in US railway slang.

https://en.wikipedia.org/wiki/Smoke_deflectors

And here is a computer model of airflow around a steam locomotive with and without smoke deflectors:

[Just thinking (OP)]

I didn't know what smoke deflectors were, so I had to look it up:

There is some mystery to smoke deflectors that were fitted to many steam locomotives first mystery who invented them and second why I know many good reasons have been explained here already but the real reason for them seems to be overlooked. The first mystery of who that is which country invented them will stay a mystery but the main reason for the deflectors is to assist in the preservation of life that is to get the crew through long tunnels without being overcome by smoke and soot as well as hot steam. Evan. AU has provided a good video here that shows just how the deflectors work they generate a long slip of air that is smooth and stable along the sides of the train so when the train is traveling through a tunnel the exhaust will be forced to stay above the train opposed to swelling down the sides. So the use of smoke deflector is rather not necessary for a train that is in service on a line that has no tunnels but they were fitted to most locomotives that did have tunnels on their line they were seen more and more often even on trains that did not need them as locomotives where swapped from service to service.