Subway circuit is a term that was created before 1914 to define a group of legitimate New York City theaters featuring shows passing out from Broadway and usually prefacing their "going on tour" or used to get a try-out of a show before being sent to Broadway for a verdict. The theater also had to be reachable by New York City subway.
Unlike vaudeville managers, who cut salaries of actors when they took the "small time", or entered houses of lesser importance, there were no cuts made in the salaries of the legitimate actors playing the "Subway Circuit". To the actors, four weeks of some subway before or after going on the road was a boon, for the theaters were all within 30 minutes of Broadway and after a performance, they were within easy reach of the theatrical clubs in the theater district.
The number of theaters part of the subway circuit has changed over time. This is a non-exhaustive list