Background Frontline railway workers have provided an essential service throughout the COVID-19 pandemic. Although railway workers are known to be at risk of developing traumatic stress-related conditions, little is known about the effects of COVID-19 on the mental wellbeing of railway workers in the UK. We tested two hypotheses: (1) increased scores in COVID-19-related risk factors will predict decreased mental wellbeing and (2) protective factors will mediate the relationship between COVID-19-related risk factors and mental wellbeing. Methods The study consisted of a cross-sectional online survey exploring the effects of COVID-19 on the mental wellbeing of railway workers (n=906) in the UK, using data collected during third lockdown (between Dec 1, 2020, and March 24, 2021). The survey, advertised on social media and internal platforms of the Associated Society of Locomotive Engineers and Firemen, included measures of COVID-19-related risk factors (COVID-19 Perceived Risk Scale, COVID-19 Stress Measure, COVID-19 Burnout Scale, and PTSD Checklist-Civilian Version) and protective factors (brief resilience coping, team resilience, general help-seeking questionnaire) associated with mental wellbeing (Warwick Edinburgh Mental Wellbeing Scale-Short Form). We analysed responses via multiple regression and parallel mediation analyses. Open-ended questions, exploring railway workers' views of risks and protective factors for their mental wellbeing, were analysed using content analysis. The institutional ethics committee granted ethical approval and participants gave informed consent. Findings COVID-19 related risk factors negatively predicted wellbeing (r 2 =0•50, F=273•556 [3 variables, n=813], p<0•0001). Protective factors significantly mediated the relationship between wellbeing and burnout (indirect effect [IE] -0•0252, 95% CI -0•0364 to -0•0152), stress (IE -0•0252, -0•0364 to -0•0152), post-traumatic stress disorder (IE -0•0311, -0•0493 to -0•0145), and risk perception (IE -0•0244, -0•0411 to -0•0089). Content analysis found selfreported mental health decline throughout the COVID-19 pandemic, perceived loss of workplace supports, need for psychological support, and concerns for the future.Interpretation These findings emphasise the importance of adaptive coping and team-based resilience in alleviating the negative impact that COVID-19 has on railway workers' mental wellbeing and in preventing burnout. Work-based supports and access to psychological input for railway workers who need it is highlighted. Building a resilient railway workforce moving forward requires attention to staff mental wellbeing and to ensuring that support systems are robust and accessible.