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 ...

For those working in the N scale, there’s now an easy way of observing your project as the train driver would see it: [Vassily98] managed to squeeze a wireless camera into an N-scale railcar.