What is it?

That little "miniature exhaust stack" was the booster exhaust on the real NYC Hudsons.

Y'know, I've always wondered the same thing about my 783.  Now I know.  Thanks.

Interesting. I just figured on some of the old "toy" trains like Marx and the lower class Lionel stuff that some of the "details" were just stuff they stuck on the locos to fill space.

Marx would not have spent the money if there was not a reason for it.

Steve

Thanks for asking the question.

My 1666E also has the booster exhaust.

Glad to see I'm not the only one who did not know

 Lionel's 226E and 1666's had this detail also.

Now that I know it is a booster exhaust, what is a booster exhaust or booster.

Unlike diesel locomotives whose pulling capacity is measured in horsepower (a unit of power), a steam locomotive's pulling capacity was measured in tractive effort (a unit of force). Tractive effort is a measure of the pulling force that a steam locomotive could apply from a stationary position. Generally speaking, if a steam locomotive could get a train moving from rest, it could pull it any reasonable speed. The amount of tractive effort that a steam locomotive could produce was determined by the geometry of the steam locomotive. In order to increase the pulling force beyond what the geometry of the steam locomotive could produce, boosters were sometimes added to the locomotive. A booster could either be added to the leading truck of the tender or more often, to the trailing truck of the locomotive. Since boosters were only capable of operating a lower speeds, they would either limit the locomotive to switching duties or must also have the ability to be disengaged which is the case with rear locomotive truck boosters.