The railway track sign may be used in chest radiography or CT to refer to the appearance of contrast in the pulmonary artery surrounding a partial filling defect in the setting of acute pulmonary embolism.
Acute embolus sits centrally and is surrounded by contrast media on CTPA. When such vessel is viewed in longitudinal section hyperattenuating contrast streaks along periphery with central hypoattenuating thrombus resemble railway track.
It should not be confused with tram-track sign.