State Line Tower was build by the Chicago & Western Indiana Railroad in 1897. The interlocking sat on the Illinois and Indiana boarder, hence the name State Line. The tower was of brick and concrete and had a slate roof. The tower was 100 feet long and 16 feet wide. It housed a 224 mechanical lever machine made by the National Switch & Signal Company of Easton, Pa.
At the time of installation in 1897 the machine had 136 levers in service, 20 spare levers and 68 spare spaces. The locking bed was of vertical type and was on the front and back of the interlocking machine. The locking bed had over 900 locking dogs and had 2,500 feet of locking bars. Almost 62,000 feet of pipeline was used and 109,000 feet of wire used. The interlocking operated 62 signals, 68 switches, 70 facing point locks, 1 movable point frog, and 39 crossing bars. The longest pipeline for a signal was 2,492 feet and 1,242 feet for the switch.
In 1910, 24 of the spare levers and spaces were put in service for the Kensington and Eastern Railroad which was expanding its tracks through the interlocking. Through the years tracks where removed and slowly the interlocking plant saw less and less traffic. The mechanical pipelines started being replaced by electric signals and electric switches. By 1985 the C&WI tracks had been removed and the IHB, B&OCT, CSS&SB, and the NYC&STL where the only railroads left at the interlocking. By the 1980's State Line Tower was under the ownership of the Indiana Harbor Belt Railroad. The tower was closed on August 5th, 2000 and was tore down the following October. Nothing remains at the site of the tower today, not even a hint of a tower foundation.