The first interlockings were strictly mechanical. The switches and signals were operated using large levers in the tower, and were connected to the levers through a series of movable pipes. During the early 1900's, electric and electric pneumatic interlockings became more popular for new construction.
When an interlocking involved multiple railroads, typically the "junior" railroad would assume responsibility for maintenance and staffing of an interlocking and tower. That is, the new railroad whose construction across the existing railroad creates the need for an interlocking, will assume responsibility. Sometimes a formula would be created for shared responsibility. Sometimes for various reasons, the responsibility will be transferred to the other railroad.
During the mid and late 1890's, many railroads operating within Chicago agreed to elevate their rights of way. In addition, some railroads eliminated crossings with each other. One notable grade separation project was at the appropriately named Grand Crossing, where the New York Central and the Pennsylvania Railroad were elevated over the Illinois Central. That project was completed in 1912, prior to then the crossing had no interlocking. In recent years, there has not been a railroad grade separation project in Chicago for a long time, until 2014. That year, construction of an overpass was completed at Englewood for Metra's Rock Island District, allowing the Metra trains to pass over Norfolk Southern's former Pennsylvania Railroad line.
The simplest kind of interlocking is where two railroads cross at grade, with no switches. Mainly around the 1940's through 1960's, many of these simple crossings mostly in outlying areas were converted from tower control to automatic interlockings. Routes would automatically be lined up, on a "first come first served" basis. In addition during that era, some interlockings went to remote control. Either as part of CTC (centralized traffic control) systems, or controlled from other interlocking towers.
Around the 1980's, Conrail and other railroads undertook numerous CTC projects and abandoned a number of railroad segments, resulting in greater efficiency and profitability for the railroads. This resulted in the closure of many of the remaining interlocking towers.