I'm not sure about other fields, but I will add radiology. A lot of research-oriented programs give residents "academic days". Some are more generous than others, but there is an institution in mind that gives a resident 0.5 days per week (or 1 full day every other week) to devote solely to doing scholarly things (education/reading, research, etc.).
Even if residents are not granted this, pursuing research starting your R2 year (PGY-3) becomes very feasible given the usually regimented schedule.