There are many scenarios in which a city might need a new metrorail system. In some cases, a new metrorail is needed to ensure dependable public transportation for a growing population. For example, the decades-old system currently in place may be inadequate to meet increased public demand. The outdated technology used in the original construction of the current metrorail may be leading to regular equipment malfunctions, raising significant safety concerns.

Another scenario may be that the existing rail system officially surpassed capacity five years ago, and the owners have struggled to accommodate the swelling ridership ever since. Construction plans for the expansion of the business district may have already been approved by the city council, and is forecasted that this expansion will lead to a rise in public transportation needs. Ad hoc attempts by rail managers to address increased usage may include running trains at faster intervals, and repairing decommissioned trains to increase passenger capacity. However, these methods often only create strains on the physical integrity of the system, resulting in significant numbers of trains breaking down. While ridership numbers would likely remain high, public sentiment towards the existing metrorail structure would likely decline due to delays caused by frequent repairs and equipment failures. These breakdowns would cause worries to rail engineers, who often express their concerns about the overall safety of the current system.

While there are those who commonly advocate merely revamping existing railways, the extent of the necessary repairs often make it more reasonable for a city to build a new system altogether. Significant engineering advancements have been made since the construction of most current rail systems, making the costs of a new railway comparable to the long-term costs of major repairs. Improved materials and design allow for the construction of an efficient, safe, modern railway.

The creation of a new rail system allow for the possibility of expanding the ridership base. As housing prices within the cities rise, locals often turn to inexpensive housing options in nearby suburbs. Without viable public transit options, these workers must commute by car into the city, creating both high premiums for parking spaces and a significant increase in traffic. This could also have potential repercussions on local government-operated lots, as well as places like museums and entertainment venues which can no longer offer free parking to visitors. Tourism numbers may decline as the lack of parking and heavy traffic complicate day trips into cities. By extending rail lines out to suburban areas, commuters will be able to rely on trains to reach their jobs, freeing up city streets and nearby highways.