Introduction Increasing children’s cycling to school and physical activity are national health goals. The objective was to conduct an RCT of a bicycle train program to assess impact on students’ school travel mode and moderate to vigorous physical activity (MVPA). Study design Pilot cluster RCT with randomization at the school level and N=54 participants. Setting/participants Fourth–fifth graders from four public schools serving low-income families in Seattle, WA in 2014 with analyses in 2015–2016. All participants were provided and fitted with bicycles, safety equipment (helmets, locks, and lights), and a 2 to 3–hour bicycle safety course. Intervention The intervention was a bicycle train offered daily (i.e., students volunteered to cycle with study staff to and from school). Main outcome measures Time 1 assessments occurred prior to randomization. Time 2 assessments occurred after 3–5 weeks of the intervention (i.e., during Weeks 4–6 of the intervention period). The primary outcome was the percentage of daily commutes to school by cycling measured by validated survey. MVPA, measured by accelerometry and GPS units and processed by machine learning algorithms, was a secondary outcome. Results For two separate adjusted repeated measures linear mixed effects models in which students (N=54) were nested within schools (N=4), intervention participants had: (1) an absolute increase in mean percentage of daily commutes by cycling of 44.9%, (95% CI=26.8, 63.0) and (2) an increase in mean MVPA of 21.6 minutes/day, (95% CI=8.7, 34.6) from Time 1 to Time 2 compared with controls. Conclusions A pilot bicycle train intervention increased cycling to school and daily MVPA in the short term among diverse, inner-city elementary school students. The bicycle train intervention appears promising and warrants further experimental trials among large, diverse samples with longer follow-up. Clinicaltrial.gov identifier number NCT02006186; Date of registration: December 4, 2013.