Wednesday, March 13, 2024


Double slip switches are known as “puzzle switches” for good reason.  A double slip switch compresses two opposite track switches into the space ordinarily occupied by a single switch.  A double slip switch has two sets of points (the part of a switch that moves) and two frogs (where one rail crosses “over” another with a gap for the wheel flanges).  The result looks very complicated.  Prototype railroad and model railroad crews are often confused as to which route through the switch has been selected/set.  

Double slip switches are rare in the prototype railroad world as track engineers usually have sufficient length or distance to separate the switch functions.  Railroads install double slip switches when they run out of room for multiple single switches or in complex terminal trackage where a double slip switch helps make smooth crossings of multiple track lines.  Most prototype double slip switches are controlled by switch towers or their modern equivalents in Centralized Traffic Control panels.


Model railroaders employ double slip switches for most of the same reasons a prototype railroad does.  We run out of room or need to make smooth crossings with switching of multiple lines.  Alas, I found myself in this predicament as I added a second main line connecting my Eugene Depot to my Eugene Arrival/Departure Yard (aka staging).  


My plan for track and switch control in this area has been to provide route control—functionally similar to having a dedicated tower operator.  The electronics for the route control have eluded me so far, so I have had to subject my yard crews to control panels with multiple toggle switches to control the switch machines.  The yard crews are still faced with verifying the chosen route.  I have attempted to provide route indication on the control panel track schematics, but this has been imperfect and crewmembers still want a more direct confirmation of route selection.


When I attended VanRail in September 2023, I operated on Doug Hicks’ British Columbia Railway Squamish Subdivision layout.  Doug also faced a need to indicate route selection for complex trackwork in his North Vancouver Yard.  Doug’s solution was to embed light emitting diodes (LEDs) in the track roadbed that switched with the turnout controls.  Aha!  


With enough time between operating sessions, I finally dug into the LED route indicator project.  I chose to apply my railroad standard use of blue LEDs to indicate a RR-East setting and amber LEDs for a RR-West setting.


Double slip switch with route indication.  The selected route goes from lower left to upper right as a straight route through the switch.  This switch also illustrates the use of a double slip switch in a crossing situation where one potential route crosses the nominal principal (straight) route through the switch.


I made up pairs of LEDs to install for the two routes at the end of each switch.  I soldered the diode anodes together and that joint to the dropping resistor needed with LEDs.  The dropping resistor (in the case a 1K ohm resistor) limits the current through the LEDs.  It is very easy to apply to high a voltage and current to an LED without such a resistor.  The result is a brief flash of light and then a dead LED.


LED pair assemblies.  The blue LED pairs use solid color wires while the amber LED pairs use color-stiped wires—useful for subsequent de-bugging.  Note the LED lead lengths limit the installation geometry.


I used 3mm LEDs, joining the anodes at the far end.  Shrink-wrap insulation tubing covered the opposite LED leads (legs) and the dripping resistor and LED joint.  The 3mm LEDs neatly fit into a 1/8-inch hole.  The length of the remaining independent LED leads limits the combination of roadbed depth and the spacing between the needed pair of indicator holes.   I found the LEDs depressed in their holes for several installations.  Those LEDs are visible when near the switch, but not necessarily visible from a distance such as fully along the set of switches in my primary four-slip switch installation.


LED route indication showing the conventional route selection.  The track on the right (the East Main) goes straight on that main route until diverging to the mainline (right track) as it passes the Eugene depot.  The track on the left is the West Main which proceeds toward the depot until it diverges to the left of the two main tracks past the Eugene depot.  That depot track is known as the WP siding, named for the construction company responsible for the Coos Bay Branch.


I chose to power the LED pairs through the frog power routing on the switch machines.  This power routing choice eliminated other sources of error on my part.


I await comments from my operating crew.  I may need to adjust the depth of the LEDs to bring them closer to the roadbed surface.  


Wider view of the RR-East Eugene switch throat.  Note that the LED route indicators for the two upper (further away) slip switches do not show well from this angle.  The LEDs are below the roadbed surface.  They become visible when one moves to a spot more in line with the switches.