Traditional methods like trapping and poisoning are already being used, but there are a lot of rats down there, and this could potentially slow the population in a more humane way. A sweet-salty, sterilizing chemical called Contrapest will be placed in subway bait boxes. When a female rat ingests some, it'll target the ovarian follicles, permanently sterilizing it. If it's already pregnant, some of its female children will even become sterile. Along with those more traditional techniques, Metro Transit Authority officials are hoping this puts a dent in the rodent population.