Uh, N Scale. A 18650 could (just) fit into a N Scale 40′ boxcar (~19w x 22h x 80l). Practically there wouldn’t be room inside a loco with its drive mechanism.

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.