Impact of the IRT on Morningside Heights
The chart above (fig. 11), drawn from data in the "Buildings List" appendix of Andrew Dolkart's "Morningside Heights: a history of its architecture and development" , shows residential building permits issued, by year. The dramatic surge in permits in the period around and after the 1904 IRT subway opening is clear. The dip in the middle of the spike, according to Dolkart, may be explained by the financial Panic of 1907.
Dolkart describes two main phases of the residential construction activities which took place around the opening of the IRT through Morningside Heights. The first phase was a period of "speculative building" of row-houses (homes designed as single-family homes). This first phase of construction (which was in part driven by speculation that the neighborhood was on its way up due to the arrival of large institutions, in the wake of the departure of the Bloomingdale Asylum, and the eventual sale of several plots of land owned by the New York Hospital in 1899) was not an unbridled success. Several of the row-houses went unsold, and ended up as fraternity houses or other unplanned uses. 
The second phase, which accounts for the large spike directly after the opening of the subway in fall of 1904, was a boom in the construction of large-scale apartment buildings. As Dolkart writes, this phase came as living patterns in New York City were changing:
"By the 1890s, the apartment house was becoming an acceptable choice of living accommodation for the middle class. Although no longer a novelty, apartments were not yet the most prevalent form of housing in Manhattan and they were not the only choice for affluent people" 
By the time around the subway's opening, apartment buildings were supplanting row-houses:
"By the early twentieth century, housing conditions were changing dramatically and single-famlily home construction virtually ceased. The American Architect and Building News documented the construction of just over 100 single-family dwellings in Manhattan in 1901, in comparison to the 835 that had been built in 1890" 
The opening of the subway was the factor that brought all these trends to a head:
"... Morningside Heights was transformed into a conveniently located area, less than a thirty-minute commute to City Hall, and even closed to downtown retail and the burgeoning entertainment district around Times Square. The opening of the subway thus led directly to major investment in speculative apartment house construction on Morningside Heights and the area was rapidly transformed into a middle-class apartment house neighborhood." 
A headline (fig. 12) from an article from the Real estate record and builders' guide, v. 78, no. 2004: August 11, 1906, describes in street-level detail some of the construction of "high-class" apartment buildings in the area, and discusses the potential of various areas of the neighborhood.
"... the Elevated carried the population past Morningside Heights, but left this area bare almost until the Subway electric road and its heralds began a new era."
"From Cathedral Parkway to 118th st, the Heights section is building up not only with great rapidity, but with a generally higher class of apartments that can be found in any other part of the city of equal area."
The full text of the article, in a more legible size, is available at this link.
 Dolkart, 341.
 Dolkart, 277-84.
 Dolkart, 287.
 Dolkart, 287-88.
 Dolkart, 289.
 Real estate record and builders' guide, v. 78, no. 2004: August 11, 1906, p.255. http://rerecord.cul.columbia.edu/rerecord/volume.php?vol=ldpd_7031148_038, accessed on July 22, 2015.