In the early 1900s a mounted signalman known as a West Side cowboy warned traffic and pedestrians on 11th Avenue when a train was approaching. 

At the time, Eleventh Avenue was known Death Avenue due to the grade level New York Central freight line, which killed many pedestrians trying to cross between trains. The cowboy led the train through the crossings with a bright red flag and lantern, alerting pedestrians of the approaching train. 

In 1929 the last train traveled up the Avenue followed by the closing of 105 street-level crossings, signaling the construction of the elevated track that would eventually become the High Line. 

Also check out this similar photo from 1908