Would it be fair to say that the primary reasons this might not be so easily done in the 21st century are economic? Lower labor costs probably had a significant impact on the ability of municipalities and private companies to build systems, roads and bridges.

Probably not fair if you factor in inflation. Today's high labor costs will look just as much a bargain in the rear view mirror as the low labor costs of 1992, 1972, 1952, and so on.

Oh if we could only pay for yesterday's labor with today's inflated dollars and incomes. And Mr. Greenspan might be right for once when he sees higfher inflation ahead.

The GNP of the USA, inflation adjusted, is 25 times greater today than it was in 1910.

Labor is labor, whether paid four silver dimes per hour or $21/hour (depending upon skill level of course).

The USA, with a much smaller population, also had much less labor available AND the only methods known in 1910 required MUCH more labor than modern methods.

In 1910, holes were dug with pick & shovel. Backhoes today.
In 1910, crowbars and gangs of men adjusted track, Hydraulic jacks today do it faster with fewer men.
in 1910 concrete was mixed by hand on site and spread by hand. Today we use concrete mix trucks and concrete pumps.
In 1910, supplies came in via mule drawn wagons, today much faster & larger capacity trucks (tomorrow battery trucks from the rail served warehouse ?)
In 1910, wooden ties were the only choice. Today, concrete is the most common choice, with composite (recycled plastic) ties as common as wood.
In 1910, jointed track was bolted together, today it is welded (much less labor, easier on rolling stock).
Etc. Etc, Etc.

In the turn-of-the-century era, the USA devoted a significant social & economic effort to build Urban Rail. A MUCH lower effort today could do MUCH MORE today. That is the force multiplier of technology and capital.

Best Hopes,

Alan