Slow Horses season 4 is set to expand the story of Slough House, yet images from the upcoming season suggest that the show is revisiting one of its main character’s lowest moments. Like every other agent in Slough House, River Cartwright finds himself in the undesirable MI5 offshoot because of some previous professional impropriety. However, while his colleagues’ indiscretions are firmly in the past, details shared ahead of season 4 suggest that River’s trauma is about to be revisited.
https://screenrant.com/slow-horses-season-4-scene-river-train-callback/