*3.2. Underground Infrastructure Network: Metro System*

Based on TMB confidentiality policies, this research does not offer Metro system details such as ventilation grates, accesses, tunnel dimensions, incoming water volumes, and ridership flows. The system runs by one operator (TMB) whose fare integrates into the unified six-zone system created by the Metropolitan Transport Authority. This research has TMB support, manager of the Metro system as a benchmark public mobility company in Europe and the world. With 13 lines, consisting of ten Metro lines, a cable car, a tram and a funicular, with a length of 121.4 km for the entire network, and 160 stations, the Metro network is the second largest conventional Metro network in Spain after Madrid, and since December 2009, the first with automated lines.

In 2019, 411.95 million passengers used the Barcelona Metro network managed by TMB. Barcelona Metro Line 3 links Zona Universitària and Trinitat Nova stations along a 17.8-kilometre double-track stretch with 26 stations underground. Line 3 at the peak hour averages 26 trains with 95 million passengers being second in terms of ridership volume, reaches a commercial speed of 26.5 km/h, and uses a rigid catenary system for the power supply and standard track width [60]. Figure 8 reveals flooding events' marked concentration on one specific area of Metro stations, which coincides with the most frequent surface pluvial floods area; during rainfall events, stormwater flow exceeds sewer pipe capacity.

(**b**)

**Figure 8.** (**a**) Location of the stations on Line 3 of the Barcelona Metro system; (**b**) Barcelona Metro stations affected by flooding in the last 20 years, the lines to which they belong, and the number of service interruptions due to flooding at stations. The size of the nodes for every affected Metro station is proportional to the number of disruptions and related to the geometric element.
