He explains it in his own words in the demo video. The sensor was small enough for an N-gauge train, which translates to 1:148 scale or about 9mm from rail to rail. His idea was to build a tiny ...

the track in N-scale layouts is 9 mm wide, meaning that the whole system had to fit in just 23 x 20 mm 2, the frontal area of a typical train car. One of the few cameras that fit within that ...