One of the structures most often seen along the right of way was the interlocking tower. These were most often located at busy interchanges and at places where two different rail lines crossed. Their purpose was to house equipment that allowed the tower operator to control switches, derails & signals to direct traffic through the junctions.
A few years ago, I did a couple of posts about the tower in Deshler, Ohio. This was an important point on the B&O mainline as there was a diamond and three interchange tracks located there. There were additional passing tracks and sidings as well. A lot of the more complex track work has since been removed but the diamond and interchange tracks remain.
The equipment in the tower was operated by hand in the early days. Levers connected to switches, derails and semaphores were moved by the tower operator to align tracks properly for oncoming trains. The term interlocking came about as the levers were connected in such a way that if one was thrown to a certain position, others couldn't be moved into a position that would cause a problem. For instance, if a switch were set to allow a train to take a diverging route, the switch at the other end of that track couldn't be set to not allow that train to pass. As time went on, power equipment allowed for some machines to move switches and other devices with much less effort. Multiple switches & signals could be set with one lever. Finally, with the advances in electronics, the control could just as easily be done remotely so the tower operator's position... and the towers themselves became obsolete.
While many railroads had "standards" to which these towers were to be built, there was a lot of variety & diversity among the structures. This was often dictated by the size of the junction and the availability of various construction materials. To see this variety, I'd suggest a visit to: http://northamericaninterlockings.com/index.html Here you will find a large collection of photos and a description of how the towers operated.
In the next couple of posts, I'll share my efforts in kitbashing a tower on stilts and share with you some photos of another tower built by a friend.