The 44-ton GE diesel was on the higher end of this spectrum, and they did have their applications--many were purchased by electric lines switching to diesel power, and on the West Coast, 44-tonners took the place of GE and Baldwin-Westinghouse steeplecabs on lines like the Sacramento Northern, Central California Traction and Tidewater Southern. While these engines were later supplanted by heavier roadswitchers, they did provide mainline service for decades, pulling the same short trains the electrics had.
Some 44-tonners are still in use today--the Sacramento Southern, the CSRM's museum/exhibit line, uses a 44-tonner (along with an SW-1 and some other antique diesels) to switch lumber from the UP to a commercial customer reached via the museum line.
Smaller "critters" were used in all sorts of applications--narrow-gauge gas critters were used in World War I as a compact "trench railroad", many of which were sold as surplus to industrial lines. The Grandt Line 23-ton and 25-ton diesel boxcabs certainly count as "critters" and there are many other examples out there--mostly homebrew jobs, as many "critters" were not standard offerings!
