Tram Track Sign on Chest CT
The "tram track sign" on chest CT refers to parallel linear opacities representing thickened, dilated bronchial walls seen in cross-section, most characteristically associated with bronchiectasis.
Radiologic Appearance
The tram track sign appears as two parallel lines running alongside each other, resembling railroad tracks. This occurs when:
- Bronchial walls become thickened and inflamed
- The airways are viewed longitudinally (lengthwise) on CT imaging
- The thickened walls create parallel linear densities with the airway lumen between them
Primary Clinical Association
Bronchiectasis is the classic condition associated with the tram track sign 1. In a recent study of 130 patients with CT-confirmed bronchiectasis, the tram track appearance was identified in 35.4% of cases on chest X-ray, though it had relatively poor interobserver agreement (kappa 0.256) 1.
Important Caveat
While traditionally considered a hallmark feature of bronchiectasis, the tram track sign is not the most sensitive finding. The same study found that more subtle features like granular shadows (88.5%) and vague cardiac silhouettes (48.5%) were actually more frequently present 1.
Other Contexts
The term "tram track" has also been used in orbital imaging to describe optic nerve sheath meningiomas, where the optic nerve appears as a linear defect through an orbital mass 2. However, this is a completely different anatomic context and should not be confused with the chest CT finding.
Clinical Implication
When you identify parallel linear opacities (tram tracks) on chest CT, actively search for other confirmatory signs of bronchiectasis, including:
- Ring sign (dilated airways in cross-section)
- Signet ring sign (dilated bronchus larger than adjacent pulmonary artery)
- Lack of normal bronchial tapering
The presence of tram tracking should prompt consideration of underlying causes of bronchiectasis and appropriate workup for chronic airway disease.