Little is revealed about Railston's personal life, other than by Series 3 she has entered into a romantic relationship with DS Steve Arnott.
Railston and DS Steve Arnott are revealed to be in a relationship, meeting eachother after work in a police station car park and discussing celebrating their one year anniversary.
Sam visits DS Steve Arnott whilst he is being held in custody, after being arrested by DI Matthew Cottan on suspicion of murdering Lindsay Denton. She is only able to meet with him due to the fact she is friends with the Custody Sergeant at the station he is being held, and can only meet for two minutes.
She tells him that since he has been arrested, she has also been treated with suspicion by her colleagues, with her DCI putting her on desk duty and outright accusing her of colluding with Arnott in Denton's murder. Due to the pressure in their relationship, which has only been exacerbated by Arnotts arrest, the two part ways.
Sam reappears when she and DC Antonioni are charged with taking over the crime scene investigation at the Huntley residence to ensure the legal defense cannot argue that officers from DCI Roz Huntley's station could bias the investigation. This is much to the annoyance of DS Neil Twyler.
Railston later appears alongside Detective Chief Superintendent Lester Hargreaves from the Murder Squad as they interview Nick Huntley whilst he is in custody at 4th Street Station. Her and DS Steve Arnott share an awkward reunion in the station lobby, not before DS Kate Fleming admonishes her for abandoning Steve at his time of need. She later visits DCI Roz Huntley, accompanied by DC Antonioni, where Huntley lies about witnessing her husband washing a navy jumper the day before.
Under the direct orders from ACC Derek Hilton, Railston and her Murder Squad colleagues are partnered with DC Jamie Desford and other AC-9 operatives to take over the investigation from and undermine the efforts of AC-12. She accompanies Desford and Hilton to Queens Chase Woodland, where authority of the crime scene established by Superintendent Ted Hastings is officially transferred to them. After damning evidence emerges, she and DC Jamie Desford proceed to arrest DCI Roz Huntley, who demands they take her to the AC-12 Building.
Now working in the Serious and Organised Crime Unit with her boss DCS Lester Hargreaves, Railston is shown attending the aftermath of a raid. Conducted by the Strategic Firearms Command as part of Operation Strand, the raid resulted in the arrest of Paul Slater and his biker gang, and the seizure of a large amount of heroin.
From a separate office, she watches the live footage of DCS Lester Hargreaves and DS Immerton interviewing Slater, who denies hijacking the ED-905 convoy, and states he is being set up by the rival Organised Crime Syndicate. She is then joined in the viewing by her ex, DS Steve Arnott, who is investigating the convoy hijacking for AC-12.
She is later also seen attending the aftermath of a second police convoy attack by the Organised Crime Syndicate outside of the Midlands Forensic Ballistics Laboratory, where she is also met by DS Steve Arnott. Afterwards he gives her a lift back to her house at Margarita Plaza. Before leaving, she apologises to Arnott for breaking up with him after he was framed by DI Matthew Cottan in relation to the AC-12 investigation of DI Lindsay Denton, and says they should catch up. Arnott says he will call her, and she kisses him on the lips before getting out of the car.
Working under DCI Patel, Railston is part of the Serious and Organised Crime team sent to process the crime scene of the abandoned truck that was used during the robbery of the Eastfield Police Storage Facility. Meeting with DS Steve Arnott, Railston, unaware that her boss DCS Lester Hargreaves was shot dead by John Corbett during the robbery, explains that everyone is talking about Hargreaves going missing and asks Arnott what he knows. Despite knowing, Arnott tells her for operational reasons he is unable to say.
After Roisin Hastings is attacked and tortured by DS John Corbett, Railston and DS Immerton are dispatched to the hospital she is taken to in order to investigate the incident. Railston has two Authorised Firearms Officer's from the Strategic Firearms Command stand guard outside the hospital room. When AC-12 arrives and Superintendent Ted Hastings immediately goes to see his wife, Railston informs DI Kate Fleming and DS Steve Arnott that Roisin didn't want Hastings involved, hence her presence. She remarks to the two that she is still processing the fact that her former boss DCS Lester Hargreaves was corrupt, and that people have been insinuating that she should have known.
Before the trio leave, she asks Superintendent Ted Hastings if he is aware of the significance of the location of his wifes injuries (caused by John Corbett drilling into her arms, knees and ankles), to which he responds no. She then asks DS Steve Arnott to call her when he can.
Railston meets Steve Arnott for a drink after work. She tells him she is glad that he called, because she thought he would still be "pissed off" with her (for having left him whilst in custody falsely accused of corruption in season three). She opens up to him about the difficulty of discovering that her ex-boss Lester Hargreaves had been corrupted. She asks Arnott if AC-12 plan to open an investigation into the whole of Serious and Organised Crime to see if Hargreaves acted alone, but Arnott concedes AC-12 do not have the resources. Arnott tells Railston that he is no longer pissed off with her, and she goes back with him to his flat.
Back at Arnott's flat, they have a sexual encounter. She climbs on top of him but senses something is wrong (for Steve is suffering from back pain related to his fall in season four). Heavily medicated for his back-pain it is implied that Steve is unable to get an erection, and he asks her to stop. Feeling humiliated he locks himself in the bathroom. She asks Steve what is wrong through the door, but he is defensive and gives nothing away. As Railston heads to leave, Arnott begins to cry against the bathroom door. Alone, he goes to the kitchen to take another dose of painkillers for his back pain.
Later, she meets Arnott again along with Kate Fleming whilst in charge of a crime scene at McDade & Company Breakers Yard in which the bodies of John Corbett and Jackie Laverty have been found dumped by the Organised Crime Syndicate. As they approach Kate Fleming tells Steve Arnott that she thought he was getting back together with Railston, but Arnott replies back to her unequivocally that they are not. Sam tells Arnott and Fleming that an OCG vehicle was spotted on CCTV in the vicinity. She watches on as forensic scientists open the bag in which the bodies are wrapped and uncover the dismembered body parts.