Type M Submarine
Type M was designed as a 'coastal submarine', intended to keep enemy ships away from own ports. Another design goal was to allow it to be transported by rail, which was achieved by breaking the hull into parts each of which was small enough to fit on a railway car. The parts could be reassembled in any shipyard, even civilian one. This allowed the subs to be quickly transferred between naval theaters and even appear in places where submarines aren't usually expected (like large lakes).
Type M only carries 2 torpedoes, it needs to go back to base in order to reload. It also has a 45mm gun, which can be used as a (very poor) anti-air weapon, but is mainly intended to shoot at small enemy boats which can not retaliate.