Choosing Switch Machines

Choosing Switch Machines from lots of different types of electrical switches can be used in wiring your model railroad.

Lots of different types of electrical switches can be used in wiring your model railroad. It can often be quite confusing trying to decide what kind of switch to use when. For example, what is the difference among the SPST, DPST, SPDT or DPDT varieties? What do all those letters mean? Many modelers like to use the Atlas switches for their control panels to try to simplify the process, but even these are confusing sometimes. How do you know when to use the Atlas Selector, Connector, Twin and Relay Switches?

Single Pole (SP) vs. Double Pole (DP): The difference between single pole and double pole electrical switches is the number of items that the switch can control. If you use a single pole switch, it will control one accessory. A double pole switch can control 2 accessories.

Single Throw (ST) vs. Double Throw (DT): -The single throw electrical switch is an “on-off” type switch. One flip of the switch turns the light on, Flip the switch the other way and it turns off.

-A double throw switch is an “either-or” type switch. You have either one light on or another but not both. Or you have either one section of track powered on or another but not both at the same time. In other words, if you flip the switch one-way, Light A comes on and Light B goes off. Flip it the other way, Light B comes on and A goes off.


SPST – Turns one light (or track) on or off.

DPST – Turns two separate things on or off using one switch.

SPDT – One way turns A on and B off. Flip the switch to turn B on and A off.

DPDT – One way turns 2 things on and 2 other things off. The other way reverses this. This can be used to control track polarity of an isolated section of track. One way turns Rail A positive and Rail B negative. The other way turns B positive and A negative.


Layout Preparation for Choosing Switch Machines

Switch machines are generally designed to mount under the layout with the spring wire transmitting the linear motion through the layout board and roadbed to the turnout throwbar.    Turnouts should be checked for free movement of the points and a clear area under the layout for mounting the machine.  Although the switch machines are extremely forgiving in mounting, a turnout that binds will still not switch smoothly.  It will be necessary to provide a hole or slot directly under the throwbar.  This hole is typically located between the rails, but may be outside the rails, if desired.

The size of this hole should equal twice the total turnout throw (1/4” works well for HO and S).  In addition, a small hole will have to be drilled in the throwbar for the spring wire to pass through.  Some turnouts have a rivet in the throwbar which can be used for this purpose. Next determine the total thickness of the roadbed and board and mark the 1/4” bit with tape or use a drill stop.  Then, using the pilot hole as a guide, drill up through the base and roadbed, being extremely careful not to break through and destroy the throwbar.  It may be possible to flex the throwbar away from the roadbed slightly to help prevent this.  Finally, use a hobby knife to remove any remaining roadbed and to clean up the hole.

Choosing Switch Machines

Choosing Switch Machines Preparation (Tortoise)

The spring wire which is usually provided with the switch machine is suitable for layouts having a total roadbed/baseboard thickness of one inch or less.  If your layout thickness is greater than 1”, you will have to substitute a suitable length of .025” spring wire for the one provided.    If greater tension on the points is desired (for O Scale and larger), you may substitute a heavier gauge wire.  The output arm will have to be drilled out with a suitable diameter bit held in a pin vise. Do not use a power drill!

  1. Prepare the spring wire by gripping it with pliers 1/8” from one end and make a sharp 75-80 degree (nearly right angle) bend. At a point 3/4” away from the initial bend, make an additional 15-degree bend in line with the first.  Figure 1 is full size and may be used as a guide.
  2. Using a #1 Phillips screwdriver, carefully thread the small Philips head retaining screw into the large hole in the throw arm.  Do not push any harder than necessary on the arm!    After the screw bottoms out, remove it.    It will be reinstalled after the switch machine is mounted.
  3. Slide the fulcrum into the fulcrum rails on the front face of the switch machine so that the larger openings of the tapered pivot holes face the bottom.


Cut out the template and use a punch or hobby knife to make a hole in the center of the large black dot.  The switch machine can be mounted off centerline if necessary for special clearance applications, in which case one of the two dotted alternate circles should be used.    (This changes the geometry of the spring wire, though.)    Also, Tape the template to the underside of the layout with the hole you just made centered over the 1/4” hole previously bored through the layout.  NOTE:  The large arrow should be parallel to the rails.  Drill pilot holes for #4 wood or sheet metal screws at the four points indicated (3/32” or #42 drill bit).  Remove the template and fasten the switch machine to the layout with #4 x 1/2” wood or sheet metal screws.  The hole in the fulcrum should be centered directly under the hole in the layout.

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