Mimic
(1997)

Peter (Jeremy Northam) comes across the cockroaches’ nest while escaping from them. He cracks open a gas line and then strikes the floor with a pick axe, creating a spark that sets the gas ablaze and completely destroys the nest. He escapes the explosion by jumping into a flooded sewer below. Susan (Mira Sorvino) fights the fertile male and kills it by putting it in the path of an oncoming subway train.

As she and Chuy (Alexander Goodwin) are being tended to by paramedics in the street, Dr. Gates (F. Murray Abraham) arrives and tells Susan that he and others did a full sweep and nothing could've survived. Then, Peter steps out of a subway entrance alive and well, and he, Susan & Chuy hug. They are the only survivors.

Thanks Tornado Dragon and Unreality!