The metro system is the main mode of transportation in Taipei City. The air quality of metro stations is crucial for passengers. This study investigated particle size distribution and its elemental composition and assessed tunnel washing performance in the Taipei Rapid Transit System (TRTS). A 24-hour particle sampling process was performed in the tunnels, platforms, and entrances and exits to measure particulate matter (PM)2.5 and PM10 concentrations in an underground metro station and to analyze PM metal components. PM10 and PM2.5 concentrations decreased sequentially from the tunnels to the platforms and then to the entrances and exits to ambient environment in the metro station. The main metal components of suspended particulates in the TRTS mainly included iron, barium, copper, manganese, magnesium, aluminum, chromium, zinc, nickel, and lead. The total PM10 and PM2.5 metal proportions were 33.9%–24.7% and 32.9%–22.8%, respectively. Furthermore, the effectiveness of tunnel washing in reducing the PM concentration was investigated. Monitoring results showed an increase in PM10 and PM2.5 concentrations after tunnel washing on the first day. The PM concentration started to decrease from the second day. Furthermore, 3.5 months after tunnel washing, the PM10 concentration decreased by 45.9%, and 2 months after tunnel washing, the PM2.5 concentration decreased by 71.3%. The mechanism of the continuous reduction in the PM10 concentration after cleaning is probably related to the porous material of tunnel walls, which may provide a deposition sink for aerosol particles, as well as the filter effect of the air conditioning system. This is the first study to use full-section tunnel washing to reduce PM exposure at mass rapid transit (MRT) stations. Although full-section tunnel washing reduced the PM concentration in the metro station, accessorial technology, such as the air conditioning system or platform design, may help in reducing the exposure of MRT passengers.