An interlocking machine inside Tower 17. The electro-mechanical interlocker controlled
the railroad signals and switches, and only one route could be set up at any one time,
and helped to prevent accidents by allowing only one train through the crossing at a time.

The electromechanical interlocker was sort of a 19th century computer. The towerman used a
reference board which showed which levers had to be pulled in exact sequence to line up the route.
To set up a different route, the towerman had to "take down" the existing route by pushing in the
levers in exact reverse order that they were pulled. This reset the interlocking machine
and then the new route could be aligned. Although more advanced technology was connected to
the interlocking machine, it basically functioned in the same manner for over 100 years!