Throwing a Turnout

In the last issue of the Proto:87 Journal, I described how I made a detailed turnout for my Canada Atlantic Railway's Pembroke Southern branch [Gourley, 1997].  One of the features I had planned to correct on this detailed model was the grossly over-scale switch rod that is common in standard turnout construction.  As a result, the switch rod is purely cosmetic: the point blades are thrown individually by a mechanism beneath the roadbed.  This article describes the "throwbar" beneath the roadbed which allows me to use an aesthetic scale switch rod.

The distance through which this throwbar moves the the points is actually very small -- 4 1/2 scale inches or about 1.25 mm (0.05") in HO scale.  This means it is nearly impossible to tell whether the switch is open or closed by looking at it from any reasonable distance.  Running through closed switches is one of the leading causes of derailments, and so, I also describe a mechanism for a switch stand that indicates the position of the switch.  As with the throwbar, this mechanism is easiest to build beneath the roadbed.

Both mechanisms are connected to a single Tortoise slow motion switch motor.  These somewhat expensive units are easy to install and extremely reliable.  They move the points slowly and quietly, and apply constant pressure to keep the switch positively aligned. 

The Tortoise is a sealed unit that moves an actuating arm through a travel of approximately 12 mm (.5 in).  A spring wire is connected to the actuating arm and passes through a fulcrum to transmit the motion to the switch.  There are two plastic rails on the machine that allow the installer to move the fulcrum up and down to adjust the amount of motion transferred to the switch.  The motor stalls when it reaches the limit of its travel, providing constant pressure on the points.

Ordinarily, the wire passes through the roadbed and engages a hole in the switch rod.  This hole, and the force applied to the joint between throwbar and point typically require a switch rod that is more substantial than its prototype.   

There are a couple of alternatives that make for a very elegant and functional switch rod, however.  Stephen Ottaway [Ottaway 97] showed us a very fine one made from

thin double-sided PC board.  To the underside of this, he had affixed a tube which accepts the actuating wire.  Also, Doug Lowing [Lowing 97] describes a switch rod represented by a piece of rail.

Subroadbed Throwbar

I had heard of British modellers attaching a "dropper wire" to each point and moving these with a throw bar beneath the roadbed.  The cosmetic switch rod can then be as fine as required to accurately portray the real switch rod.  This approach requires two holes through the roadbed, but these holes are easily obscured because they are beneath the points themselves. 

In my case, the dropper wires are better called "dropper posts" as they are quite substantial, being made of 3/32" brass rod (Figure 1).   Lighter dropper wires may flex when passing through the roadbed, resulting in poor motion at the points.  The heavier posts hardly flex at all, and so, they can accurately move a point through thicker roadbed.  Mine is roughly 25 mm (1 in.).

As I described in [Gourley, 1997], the ends of the posts are filed to represent the clamps used to attach the switch rod to the points.  The end was then soldered to the web of the point rail, making a strong joint to resist the force applied against the stock rail. While the diagrams do not show it, the bottom ends of the posts were cut at an angle to make them easy to push into the throw bar.

The throw bar beneath the roadbed was constructed from styrene square-section tube (See Figures 2, 3).  The use

Figure 1: 3/32 inch brass posts (b) were soldered to the insides of the point blades (a).  The posts pass through the roadbed and engage a sturdy throwbar which is driven by the Tortoise switch machine.

Figure 2: The throwbar is made in two halves to allow it to match the distance between the points.  The dimensions of the base of the guides allow for a good fit with the Tortoise rails. Dimensions in millimetres (inches).