Sustainable development is a concept which has, by now, become synonymous with all aspects of human life. The term originated from the 1987 Bruntland report and is defined as: "Sustainable development is development that meets the needs of the present, without compromising the ability of future generations to meet their own needs." (source)

(Image from http://www.thesustainableleader.org/sustainable-development)

In terms of rail travel, sustainable development should involve an equal balance of:

Environmental sustainability: By increasing rail patronage, less people are likely to travel by car (or on long-distance journeys, by air). This leads to less CO2 emissions, thereby contributing to less global warming, and other environmental impacts such as air pollution.

Social sustainability: Development of rail should provide more opportunities for travel to all members of society equally. Rail should enable people to better their lifestyles through increased connectivity to employment, education, healthcare and social well-being.

Economic sustainability: Rail development should lead to sustainable business growth, not just for operating companies and transport organisations, but for local businesses and residents who are empowered by better rail links.