The Northern Illinois Railroad was built in 1884-85 to connect the coal fields of Spring Valley to Belvidere, to the north, and what was perceived as lucrative railroad connections there. Consequently, the NIRR also saw steady passenger business, as it was a main north-south link in the region. The Chicago and Northwestern Railroad purchased the line in 1888.
Passenger service ended in the 1920s. Both the northern and southern sections of the line have since been abandoned: On the north side, the Belvidere-Sycamore segment saw abandonment in 1942; Sycamore-Dekalb in the 1980s. To the south, the Spring Valley-Troy Grove line has been abandoned (year unknown). The remainder of the line continues to see use under C&NW successor Union Pacific Railroad.
Thanks to Wayne DeMunn for contributing information about this route.