The duration of Line 2A, also known as the Second Avenue Subway in New York City, is approximately 3.7 miles (6 kilometers) long. This line, which runs from 125th Street in Harlem to Hanover Square in Lower Manhattan, opened in phases between 2017 and 2021. The initial phase from 96th Street to 63rd Street opened in 2017, followed by the extension to 96th Street in 2019, and the final phase to Hanover Square in 2021.

The construction of Line 2A was a complex and lengthy process due to various factors. First, the line had to navigate through dense urban areas with existing infrastructure, which required extensive planning and coordination. Second, the project faced significant budget overruns and delays, which were partly due to unforeseen challenges during construction. According to the Metropolitan Transportation Authority (MTA), the total cost of the project was originally estimated at $14.5 billion but eventually reached $30.2 billion.

The duration of Line 2A's construction can be attributed to several key factors. Firstly, the project involved the construction of new tunnels and stations, which is a time-consuming process. The tunnels for Line 2A were bored using Tunnel Boring Machines (TBMs), which can be slow and prone to technical issues. For example, the TBM used for the 63rd Street Tunnel experienced a significant delay due to a breakdown, which prolonged the construction timeline.

Secondly, the project faced regulatory and environmental challenges. The construction of a subway line in a densely populated city like New York requires compliance with numerous regulations and permits. This process can be lengthy and complex, adding to the overall duration of the project. Additionally, the project had to address environmental concerns, such as the protection of historic sites and the mitigation of noise and vibration impacts on surrounding communities.

Lastly, the project's budget overruns and delays were significant contributors to the extended duration. The MTA faced financial constraints and had to reallocate funds from other projects to cover the increased costs. This reallocation of resources further delayed the completion of Line 2A.

In conclusion, the duration of Line 2A, the Second Avenue Subway, is approximately 3.7 miles long and took several years to complete. The project faced numerous challenges, including construction complexities, regulatory hurdles, and budget overruns, which collectively contributed to the extended timeline. According to the MTA, the total cost of the project was originally estimated at $14.5 billion but eventually reached $30.2 billion, highlighting the financial and logistical complexities involved in such a large-scale infrastructure project.