Perfectly preserved "armstrong lever" interlocking tower.

[Overmod]

PATTBAA

Fully intact since its construction circa 1880 is the "SONO Switch Tower Museum with 60+ "armstrong levers".  The technology of that time was  a"mechanical only" form of switch manipulation using levers, rods, etc.  The "interlocking" feature was a very complex mechanical design using an intricate assembly of parts to perform the interlocking function.

Keep in mind that there is a distinction, here as in the other thread involving towers, between the interlocking frame itself and the network of cranks and rods that communicate 'armstrong' movement out to the physical switches and other equipment.

The interlocking frame is the thing that survives at SONO (and Miller and some of the other mechanical tower plants that are also preserved).  The issue is that the mechanical connections from the frame out along the ROW have not, to my knowledge, been preserved in these restorations; SONO is explicitly no exception (although you can't confirm this from their present Web site and I can't access any of their picture gallery that may not be 'under construction' there is at least one video where the staff confirm the frame is not hooked up to linkage outside the building.)

The invention of mechanical interlocking is surprisingly old.  It was first commercialized in Britain very shortly after the Civil War period (I believe the Saxby and Farmer patent is from 1867) and was copied here within a decade.  The larger plants are fascinating in their complexity, and in the precision with which the locking mechanism is made to run effectively with minimal required maintenance.  

Later plants did not only 'interlock' the levers safely, they arranged to move multiple switches together simultaneously, analogous to couplers on a pipe organ, so that things like crossovers and 'route lining' as for CTC could be coordinated.  It was possible to do this with mechanical linkage, but with power assist ... usually compressed air.

Note that there was more complication in the linkage than just bellcranks, joints and rollers.  In a 'safe' plant the switches have to be positively locked as well as lined, and manual 'override' has to be provided, ideally without disconnecting the linkage or impairing subsequent 'remote' operation.  Quite a bit of 19th-Century ingenuity went into devices and methods for this.

Along with this came the development of electric signaling, the early history of which precedes the practical invention of the dynamo.  It is fascinating to see the development of signaling methods that rely entirely on battery power, with either the fluids or the cells themselves being periodically replaced -- all the early Robinson signals were made this way, and at least a couple of the original automatic train-control systems that utilized electricity.  

[PATTBAA]

1. Fully intact since it's construction circa 1880 is the "SONO Switch Tower Museum with 60+ "armstrong levers". The technology of that time was  a"mechanical only" form of switch manipulation using levers, rods, etc.The "interlocking" feature was a very complex mechanical design using an intricate assembly of parts to perform the interlocking function. A gentleman from Pa., expert in this "field", devoted a considerable amount of time " under the levers" observing the interlocking movements , and he compiled a "dog chart". (???) The tower is the subject of a book titled "The Classic Railway Signal Tower". Also in the Museum is a vintage NHRR electrification control board which is an operating exhibit.