From Wiktionary, the free dictionary

From stack (noun) +‎ talk (noun). Stack is a clipping of smokestack (“conduit atop a structure (such as a steam locomotive) allowing smoke to flow out”).

• (Received Pronunciation) IPA^(key): /ˈstæk tɔːk/
• (General American) IPA^(key): /ˈstæk ˌtɔk/, (cot–caught merger) /-ˌtɑk/

stack talk (uncountable)

1. (US, rail transport, slang) The sound made by the exhaust of a steam locomotive.

   • 1956, Railway Progress, volume 10, Cleveland, Oh.; Washington, D.C.: Federation for Railway Progress, →ISSN, →OCLC, page 23:

     […] Father [Clement C.] Kubesh began making long dashes after trains to insure the correct pitch of sound or the proper placing of his car for the approach of a locomotive. He soon had recorded the "stack talk" of every type of locomotive on the Great Northern, Chicago and North Western, Northern Pacific and Union Pacific.

   • 1956 October, “Railroad Hobby Club”, in Freeman [H.] Hubbard, editor, Railroad Magazine: The Magazine of Adventurous Railroading, volume 67, number 6, Canton, Oh.: Popular Publications, →OCLC, page 80, columns 1–2:

     [T]he priest [Clement C. Kubesh] makes highly stylized recordings of train sounds. […] To him, every steam engine whistle has a recognizable quality and each puffing stack has its own "talk." […] One October, Father Kubesh spotted a potato warehouse about a quarter-mile from Drayton, N.D., on the Northern Pacific. Plugging in his recorder, he caught No. 13, a passenger train, and recorded the whistle, stack talk, and bell, while the train was rounding a bend and while rushing by the warehouse.

   • 2022, “Norfolk & Western 611—On Home Rails—Train Blu-Ray”, in A-Trains.com‎^[1], Greenwood, Ind., archived from the original on 24 February 2025:

     Enjoy the action as this giant 4-8-4 steam locomotive pulls heavy passenger trains over Christianburg Grade and Blue Ridge Grade. […] Highlights include […] Long, heavy trains with great stack talk