I am a newbie to subway motive power but I saw a video on the R40 being used as a garbage train. Could someone please explain.

The MTA uses any motive power that is at end of life for the garbage trains. You will see any of the retiring motors on these trains The TA also has models that were specifically purchased for garbage and work trains Designations are R127 and R134

Interesting; I didn't realize their were garbage trains. 1st they used the R40 for a garbage train and then dump them in the ocean for artificial reefs.

Not exactly going out in a blaze of glory are they!

Considering how maligned the R40's were when they entered service, I am sure there are people saying that is all they were fit for in the first place and that was a fitting end (I am not saying that)..

They ran a coal train on the Myrtle Avenue line in Brooklyn. Stations were heated by pot belly stoves which burned coal and the coal train brought the fuel to the stations.