This is coolbert:
Here is another midget submarine [slightly more displacement than the conventionally accepted definition of a midget?] that saw action in World War Two [WW2]. Much more successful as a class of midget sub than any other?
The Soviet "Malyutka".
"The M-class submarines, also Malyutka-class, were a class of small, single-hulled submarines built in the Soviet Union and used during World War II."
Again, a midget sub FOR DEFENSIVE PURPOSES ONLY. The original "Malyutka" had only two torpedoes for armament. Fire those two and that was that. Back to base for replenishment!
"These boats were designed in the beginning of the 30’s as typical coastal submarines. That is why their abilities were modest enough . . . just two torpedoes . . . these cheap, mass-produced boats were intended to defend naval bases, blockade enemy harbors and, as a main feature, be railway-friendly to be transported from one war theatre to another."
Railway-friendly means the entire sub can be placed on a railway car and transported from one locale to another, where ever the need is greatest! From White Sea to Baltic. From Baltic to Black. From Black to Pacific. Etc.!
Also used modular construction. Unusual for the time - - "cheap, mass-produced boats"! YES!