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The purpose of this study was to evaluate three waterproofing systems (conventional polyethylene (PE) mats, Rockdrain, and XiiT-Foam) to determine the most sustainable maintenance solution for Stockholm’s hard rock metro tunnels. A comparative Life Cycle Assessment (LCA) and Life Cycle Costing (LCC) were conducted using a functional unit of 1 m² over a 60-year service life, combining field observations with industry data. An assumption-based harmonization approach was applied for XiiT-Foam due to limited cradle-to-grave data. The results showed both alternatives significantly outperformed conventional PE mats (GWP: 114.1 kg CO2-eq/m², LCC: 593.5 EUR/m²). Rockdrain yielded a verified improvement (GWP: 87.18 kg CO2-eq/m², LCC: 265.8 EUR/m²). While XiiT-Foam presented the lowest impacts (GWP: 1.74 kg CO2-eq/m², LCC: 159.5 EUR/m²), sensitivity analyses revealed high dependence on uncertain new-build variables like avoided-excavation credits. It was concluded that Rockdrain represents the most defensible, immediately viable sustainable upgrade for existing tunnels, whereas XiiT-Foam requires independent, maintenance-specific verification.