Inside a minimalist, airy space in the heart of New York City’s Flatiron district, I’m face-to-face with a trainer that’s about to guide myself and more than 60 other attendees through this morning’s cardio workout. Except I can’t see any of the other participants; rather, I’m staring at a slab of mirrored glass with a display panel underneath it that’s showing my trainer as she instructs the class. There are also bits of information scattered near the bottom of the glass, such as heart rate data and calories burned. That’s because I’m not in a boutique fitness studio, but in a storefront where fitness startup Mirror sells its product of the same name. Mirror — as its name implies — is a $1,500 mirror that’s meant to function as a home gym, letting owners peer at themselves in a full-length mirror as they follow along with their instructor. Mirror is just one of many companies seeking to replace traditional gyms and fitness classes with training sessions you can stream at home via internet-connected devices. Among the most recognizable companies in the space is Peloton, which sells a stationary bicycle priced at more th...