The train beat — country, rockabilly and bluegrass-with-drums. The snare plays continuous sixteenths (brushes or light sticks) with the accent on 2 and 4, imitating the chuff of a steam engine; the kick marks 1 and 3. That accented 2 and 4 inside the stream is the backbeat cell.
How it sits: the guitar plays boom-chick — a single bass note on 1 and 3 (root, then fifth) and a short chord on 2 and 4, all downstrokes — and the bass plays a two-feel on root and fifth. Everything lines up with the snare accents: the chord and the accented sixteenth land together.
Listen for the sixteenths staying even and soft; only the backbeat pokes out. The guitar chords should be shorter than the bass notes.
Practice: guitar alone, bass note then chord, with a metronome on 2 and 4; then add the drums and keep the chord strokes clipped.