Studio Daily article on the AHDN 4K restoration. Pretty technical, but it may shed some light on the Train song:
[Legendary session guitarist Vic] Flick and friends do not appear on one cue which has been baffling fans for decades — a brief rock-and-roll clip heard when Ringo flips on a portable radio while riding on a train. “That’s not me,” Flick states. So who is it? Notes Giles Martin, “My instinct says it’s not The Beatles, but more likely the session players my dad would have gotten in for the soundtrack recording.”
The track is indeed driven by Cattini ...