A train monitoring system in the present embodiment is provided with a compartment camera 10 that takes an image of the inside of a compartment of the train, a processing server 30 that calculates a vacancy probability for each reserved seat on the basis of the image taken by the compartment camera 10 while the train is running and detects a resale candidate seat by comparing the vacancy probability with a threshold value, and a touch panel monitor 40 that displays resale candidate seat information indicating the resale candidate seat that has been detected by the processing server 30.