Subway systems play an important role for metropolitan cities' inhabitants, commerce, tourism, and development. There are more than a dozen subway systems across the United States, with varying size, operating hours and regulations. While, all United States subway systems have distinct characteristics, they face similar issues. Most subway systems in busy metropolitan areas, such as New York City or Washington, D$.$C$.,$ were built several years ago. Though developments and upgrades have occurred throughout the years, many are deteriorating and fail to meet current needs. Several factors contribute to this dilemma: population increasing faster than predicted, new business development, and increasing demand for service in previously low traffic areas. To manage these demands and funding constraints, some cities are coming up with imaginative ways to better serve their customers. For instance, New York City has developed plans for trains with one long car in hopes of reducing overcrowding.

Which of the following best describes the main idea of the passage?

The distinct subway systems are more dissimilar than similar.

All subway systems face various environmental demands.

The primary concern of subway systems is handling overcrowding.

Subway systems face similar challenges despite their diversity.

A city's success is dependent on the success of the subway system.