This movie might be far-fetched, but....

A couple of years after the film was released, an interesting scientific discovery was made that "mimics" some of the features in the story: a new species of insect was found to have evolved on a subway system that preyed specifically on humans. Except in this (real-life) case, the insect was a variety of mosquito

"... closely related to the one inhabiting the daytime streets of London but that lives exclusively in the tunnels of the London Underground. Unlike their upstairs brethren, which bite only birds, members of this subterranean race show a distinct affinity for human blood."

The article goes on to say:

"After midnight, when the trains had shut down, [doctoral student] Katherine Byrne followed maintenance workers into the subway's bustling underworld, where she soon discovered an ideal environment for mosquitoes. "The temperature is warm and virtually constant," Byrne says. "There's no rain, no snow: it's great for a year-round life cycle." The polluted pools of water, ranging from "pretty nasty" to "hideous," are prime mosquito-breeding sites. As with all mosquitoes, only the females--the egg layers--seek out blood, feeding on rats and pinstriped rat-racers alike."

The full article can be found at
http://www.findarticles.com/p/articles/mi_m1134/is_1_110/ai_70770157

Who knows, perhaps the author of the story was on to something. Pretty neat, eh?