The wording is confusing about that 'platform" - it really was an underpass below the 4 tracks between the uptown and downtown local platforms. It ran under the east sidewalk of Broadway and is still there today. It was to be extended to W.41st street but never was done. I believe it was extended to be connected to the (beneath) the West Side IRT new Times Square express station 2 island platforms. It was made redundant as an "underpass" walkway when the shuttle was created -- but remained in use quite a number of years for the westbound (former N/B) platform passengers of the shuttle and its fare paying both area there, to get to the opposite E/B (former S/B) platform and make connection to the new southern west side IRT extension. Because the former N/B local "now shuttle" track was connected to the new northbound local track coming up north from the new Times Square Station of the new south extension mainline tunnel from South Ferry.
In later years and due to passenger rush hour speed, convenience and safety for crossing the 4 tracks, a quickly removable portable wide wooden platform was placed over the former N/B "shuttle" local track, to connect with a newly permanent concrete platform running across the other 3 "bumper ending" tracks of the shuttle at its own Times Square Station. Below are some photos from me of this arrangement.
BELOW -- LEFT - looking west along former N/B local mainline track at Times Square "shuttle" track # 4 back in 1958 showing the portable removable wooden platform placed across it where it connected to the 1918 built west side IRT Mainline local track. Stairway to upper level 1904 opened fare control area is seen still in use at end of platform
BELOW -- RIGHT - looking west in more recent time showing the basic same scene and more clearly is seen the trackway and steel framed and wooden deck portable removable platform across the former uptown local track, now shuttle track former # 4 track. In distance is seen the new 4 track lower west side mainline crossing / passing by / the curving former 1904 opened 4 track R-o-W original mainline which is now part of the shuttle segment. At left edge of photo is shuttle track # 3 which was the former N/B Mainline express track thru this station when it was part of the original 1904 opened original mainline.
ABOVE -- view north along the 1918 opened new northbound local track of the 1918 built lower west side IRT Line from South Ferry. The original northbound local track from the 1904 original built east side mainline, is seen entering westward and connecting to the west side new mainline local track under Broadway, The curving empty spaces between the curving rows of columns indicate where the original 4 tracks from the original 1904 mainline route from the east side curved to turn from W. 42nd Street, northward under Broadway and head north to Harlem and the Bronx.
regards - Joe F