I have a 3 speed in the back wheel of my Brompton folding bike..
50:15t, in the 349 16" wheel. now a 54:15, Mountain drive Crank
so as if a double 21.6"t" granny.

had another one in the WABAC of Childhood..
Sturmey Archer is 100 years old , but the nationality of the brand has changed.

twin top tubes is novel.. can you hang a U lock between them?

gear range can be tweaked, you could buy one of their 5 speed hubs after getting the bike too.
parts are changeable..