what is the meaning of "opportunistic houses" in this paragraph?

But this journey was extraordinary . . . it had a pioneer feel – the enormous Hudson,

with chunks of ice like broken pavement in the water;

and the huge wealthy forests, the valley full of forts and

power stations and opportunistic houses glooming on

vast icy bluffs, the railway stations more like adventurers’ huts than anything in Europe – barely manned

outposts without proper platforms, and with outlandish

names . . . Poughkeepsie, Yonkers, Schenectady; the train

high above the ground, the big wheels polishing the rails,

and the driver blowing that childishly dissonant horn.