Korea had a big ambition to be one of the most sustainable cities, so the government is paying more attention to environmental impacts as a key element in evaluating any transportation project, such as subway construction and extension. Compared to other transportation modes, the railway system provides significant environmental, economic, and social benefits, and it is considered a key factor to attain the 2050 vision of ‘European Green Deal’, which targets cutting the greenhouse gases’ net emission, achieving social equity, and ensuring the separation of economic growth from resource usage. Australia has enforced the ‘Environment Protection licenses’ in railway using special authorities that impose the respect of all environment licenses during both construction and operation stages. In Korea, despite the enforcement of environmental protection, regulations are lacking at the construction stage, where a qualitative approach is adopted to measure the environmental impacts. During the feasibility study, the Korean guideline adopts a quantitative approach and focuses on the measurement of air and noise pollution and accident reduction. Developed countries insist on considering other environmental impacts related to health, biodiversity, and wildlife (Europe, United Kingdom, and Australia).