The 42nd street Shuttle was part of the original line of the New York Subway, built by the
Interboro Rapid Transit (IRT) company and completed in 1904. The line followed the route of the current
4,
5 and
6 Trains from City Hall through
Grand Central Terminal, then across 42nd Street to
Times Square and up the
Upper West Side on Broadway, the route of the current
1,
2, and
3 Trains.
When the rest of the IRT system in Manhattan was completed in 1918, resulting in the failliar 'H' pattern of numbered trains we're familliar with today, the two north-south lines were made to be through lines and the short east-west portion under 42nd street was reduced to the shuttle that it is today.