["Journal of Regional Science, EarlyView. ", "\nABSTRACT\nThis paper examines the impact of high‐speed rail (HSR) connectivity on both inbound and outbound tourism inflows. Leveraging the opening of a new HSR line connecting a northern Spanish region with mainland Spain, we employ a triple difference‐in‐differences design combined with inverse probability weighting (IPW) to evaluate the causal effect of transport infrastructure developments on tourist trips in a left‐behind area. Using fine‐grained mobile phone positioning data for 168 municipalities over a 36‐month period, we find that while the HSR did not increase inbound tourism, it significantly boosted outbound trips among residents in municipalities with direct access to the HSR network. Our findings point to a siphoning effect, whereby improved connectivity facilitates the outflow of local tourists without attracting additional visitors. Furthermore, our analysis highlights spatial heterogeneity in treatment effects, driven by geographical proximity to HSR stations.\n"]