In theory, light rail has its advantages. It can potentially carry more passengers at a time per lane than buses, it can reduce traffic congestion under ideal circumstances and the costs per passenger are lower than with buses, assuming enough ridership. Also, light rail could be a better solution than trains when the demand isn’t high enough to merit building heavy rail.

More cities have been exploring light and commuter rail solutions in recent years. Between 2004 and 2014, the United States saw a 26 percent increase in urban route miles, with light rail and commuter rail making up the lion’s share of the increase.

Much of the recent growth has been driven by federal initiatives that have provided billions in funding to improve existing light rail infrastructure and add new urban transportation options to reduce congestion and pollution.

But the huge cost of light rail is one reason critics are challenging projects around the nation.

Some detractors note that aside from the exorbitant capital investment required, the costs of running one train — from maintaining tracks, stations and switches to employing operators and ticket collectors — ultimately run much higher than the costs of operating a new bus. These costs can be offset with enough riders, but numerous regions are reporting consistently lower rates of light rail ridership, despite upgrades to their light rail systems.

Local governments facing increasing traffic congestion continue to hope they can position their light rail systems in a way that will turn drivers into commuters. But many of the cities making the largest investments in light rail are facing big challenges, including from their communities.

The big five investors

These are the cities investing big in light rail projects right now, and some hurdles they’ve encountered.

1. Honolulu

The Honolulu Authority for Rapid Transportation (HART) is spending $8.165 billion on construction of a 20.1-mile fixed guideway system and 21 stations, with phase 1 scheduled to open in 2020. Funding $2.4 billion of the project is a state government bill that will use taxes to help close a major budget gap. Still, city planners push the system’s benefits based on predictions that by 2030 almost 70 percent of Oahu’s population — and more than 80 percent of the island’s jobs — will be found alongside the rail corridor. They also estimate the line will lead to roughly 40,000 fewer car trips on congested freeways, despite a recent 10 percent drop in rail ridership.

2. Bethesda to New Carrollton, Maryland

The $5.6 billion, 16-mile line will connect local metro lines, Amtrak service and local bus routes — assuming the legal delays ever cease. The line, and its 21 new stations, is due to open by 2022, but the schedule has been affected by lawsuits brought by environmentalists and other critics. The state transit administration says the new line will support community revitalization and reduce congestion around major economic centers. However, a U.S. District Court Judge says declining commuter numbers could affect the line’s projected ridership. His ruling to stay construction pending a new review of ridership is under appeal.

3. Seattle

The city is covering quite a bit of ground with planned light rail lines. Underway is the construction of the $3.7 billion East Link, a 14-mile line with 10 stations, due to open 2023. The East Link line is expected to connect the Eastside’s biggest population and employment centers with existing lines. An additional $1.86 billion is going toward expansion of the existing Northgate Link, with 4.3 miles of new light rail and three stations due for completion by 2021. Also expanding is the budget: Higher-than-expected costs for workers and technical challenges around bridges have driven up the price by millions. Now city planners face concern that about $1 billion from the federal government may no longer be available.

4. Boston

The $2.2 billion, 4.7-mile extension of the country’s oldest light rail system, the Green Line, also includes six stations, all due to be finished by 2020. This project is expected to reduce travel time across the line by 13 to 17 minutes, in addition to providing environmental benefits and new jobs. No word on whether the expansion will bump the line from its place as leader in light rail derailments.

5. San Diego

The Mid-Coast Corridor light rail extension, with a current projected cost of $2 billion (nearly double initial budget), includes 10 miles of track and nine stations, with substantial completion expected by 2021. The line will connect the Old Town Transit Center downtown to universities, hospitals and technology companies in the University City area, all via the Blue line trolley.

Rounding out the list

While these massive projects are bringing major changes to their cities, they’re far from the only initiatives happening. Other light rail expansions include:

1. Minneapolis - The $1.858 billion Southwest light rail line will extend the city’s Metro Green line by 14.5 miles by 2021.
2. Charlotte, North Carolinav - A $1.16 billion 9.3 mile Blue Line extension includes 11 stations and four park and ride facilities, due to open March 2018.
3. Phoenix - A $704.54 million 5-mile South Central Light Rail extension is due to open 2023.
4. Chicago - $408 million in improvements along a 19-mile stretch of the Chicago Transit Authority’s Blue Line O’Hare Branch are slated to be completed by 2019.

Time will tell whether light rail will trump buses, especially rapid transit buses, as the preferred mode of public transportation in many urban centers across America — and whether the systems being built end up justifying the sizeable local, state and federal expenditures.