Someday, there will be light at the end of the tunnels.

On two sides of Manhattan, snaking skeletons of underground rails are finally being built or extended, promising to connect two areas to New York City’s transportation grid.

On the East Side, the wait has seemed endless. The Second Avenue line has been staggering and stalling since 1929, when the city first decided it should be built.

On the Far West Side, the promise is much newer: It was only with the commercial and residential rezoning of Hudson Yards in 2005, and of the adjacent western railyard in 2009, that the decision was made to roll the No. 7 train toward the Hudson River.

Since then, silver mirrored giants, flashy developments and hip restaurants and stores have sprung up in anticipation of the extension of the train from Times Square to 34th Street and 11th Avenue.