Don't know for certain, but suspect the NYC"s chief engineer for the project , Wilgus, directed his designers to identify optimal layout of track and building columns. Given the development of air rights buildings would occur over time, studies were probably conceptual in nature, identifying reasonable structural spans, column grid patterns, spatial dimensions for interior design issues, track layouts etc. and then leaving the detailed design to each building developer/design team.
The coordination ofstructural spacing (steel frame building) with track spacing is the first step in the process, overalying the building structural grid(s) on the desired track plan ( which I imagine to be the critical/leading parameter). This process would likely require several iterations with adjustment to both building frame and track spacing to optimize track layout and potential building footprints.
This process probably resulted in a grid with spacing of 20' to 25' between columns - possibly longer depending on depth of spanning members. 25' would be very reasonable span for building as standard steel beams can manage this span easily (I am currently working on a building with 33" girders spanning 40' and which carry common beams 21" deep spanning 40', spaced 10' apart - very light weight and bouncy floor). Longer spans over irregular track geometry would require plate girders/built up beam and would be desirable to avoid because of the additional cost.
The 25' spans would probably work well with regular track layout (keeping in mind the less restrictive clearances of the day) , dimensional requirements for residential and office spaces. For instances of curving track and irregular geometry, deep plate trusses - transfer beams, would span longer distances.
The grid of columns carry the weight to bedrock. Lateral motion could be an issue and some diagonal bracing might be required but could be coordinated with track layout and building design.
This is one architect's opinion on the conceptual planning - a structural engineer will have learned opinion/knowledge about the challenges of spans and lateral loads facing the design engineer of early 20th century steel frame buildings.
OK, back to work.