“Besides piano, which instrument does Wikipedia mention in the original ‘Rein Me In’ instrumental?” The answer is saxophone.
Wikipedia describes the original “Rein Me In” as having a jazzy instrumental that includes piano and saxophone. The song first appeared as the eighth track on Sam Fender’s third studio album, People Watching, before the later duet with Olivia Dean.
That musical detail helps distinguish the original recording from the duet’s vocal concept. The song explores self-sabotage in romantic relationships, with Fender portraying someone afraid of being emotionally “reined in” and inclined to reject people who get too close. The jazz-influenced backing supports that reflective mood.
The question asks specifically about the original instrumental, not the later live arrangement or the vocal collaborators. Among the listed choices, saxophone is the only instrument Wikipedia names alongside piano. Banjo, violin and trumpet are not the instruments identified in that description.