The gap is not inflation. Adjusted to the same 2025 dollars, the entire 1932 Eighth Avenue Line, roughly 12 miles with 28 stations including West 4th Street, cost about $4.5 billion: less than Second Avenue Subway Phase 1 spent on 1.8 miles and 3 stations.
The IBX is the outlier that proves the rule. At a projected $393 million per mile it is the only current project near IND-era economics, because it runs light rail on an existing freight right-of-way instead of boring new tunnel under Manhattan.
West 4th era projects: IND Eighth Avenue Line construction cost of $191.2 million and Sept 10, 1932 opening from the New York Times ("New Line First Unit in City-Wide System," Sept 10, 1932); the 1932 opening segment (Chambers St–207th St) was about 12 miles, with a further 2.2 miles opening in 1933. Sixth Avenue express tracks cost of $22 million and April 19, 1961 groundbreaking from the New York Times; express service began Nov 26, 1967 with the Chrystie Street Connection (a separate, larger program). The W 4th–34th St segment is about 1.5 miles (author's estimate from street geometry); its per-mile figure is approximate accordingly.
Modern projects: 7 Line Extension ($2.4B, 1.5 mi, Dec 2007–Sept 2015), Second Avenue Subway Phase 1 ($4.45B, 1.8 mi, April 2007–Jan 2017), and East Side Access ($11.1B, Sept 2007–Jan 2023) figures as compiled from MTA reporting. East Side Access appears only in the duration chart: most of its spending went to caverns, terminal, and yard construction rather than route miles, so a per-mile figure would be misleading. Second Avenue Subway Phase 2 ($7.7B including debt service, 1.5 mi, groundbreaking June 2026) and Interborough Express ($5.5B, 14 mi, opening projected early 2030s) are projections, shown in the lighter shade as published without inflation adjustment.
Inflation adjustment uses BLS CPI-U annual averages (1982–84=100): 13.7 (1932), 29.9 (1961), 237.0 (2015), 245.1 (2017), and 321.9 (2025). Historical figures are nominal costs as reported at the time, not final audited costs.
Sources: Author's analysis of New York Times archives, MTA project reporting, and BLS CPI data.