Glen Bottoms at The American Conservative attempts to answer the question.
The verdict? Bottoms sees three key factors:
"1) overdesign, 2) lack of technical expertise at the overseeing transit agency, and 3) external factors like political interference and rising material costs.
Consultants retained to design these systems regularly use plans that they already possess without regard to applicability or functionality, selecting higher-speed overhead wire in rail yards and city streets or specifying certain types of rail without regard for cheaper alternatives. Excessive tunneling is also a critical cost driver."
FULL STORY: The Real Costs

Planetizen Federal Action Tracker
A weekly monitor of how Trump’s orders and actions are impacting planners and planning in America.

Chicago’s Ghost Rails
Just beneath the surface of the modern city lie the remnants of its expansive early 20th-century streetcar system.

Amtrak Cutting Jobs, Funding to High-Speed Rail
The agency plans to cut 10 percent of its workforce and has confirmed it will not fund new high-speed rail projects.

Ohio Forces Data Centers to Prepay for Power
Utilities are calling on states to hold data center operators responsible for new energy demands to prevent leaving consumers on the hook for their bills.

MARTA CEO Steps Down Amid Citizenship Concerns
MARTA’s board announced Thursday that its chief, who is from Canada, is resigning due to questions about his immigration status.

Silicon Valley ‘Bike Superhighway’ Awarded $14M State Grant
A Caltrans grant brings the 10-mile Central Bikeway project connecting Santa Clara and East San Jose closer to fruition.
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Planning for Universal Design
Learn the tools for implementing Universal Design in planning regulations.
Caltrans
City of Fort Worth
Mpact (founded as Rail~Volution)
City of Camden Redevelopment Agency
City of Astoria
City of Portland
City of Laramie