Dead-Rail Conversion of an O Scale MTH 2-Rail SP AC-6 Cab Forward, 4-8-8-2 (MTH 22-3709-2)

Introduction

I purchased this beauty on eBay (from Australia!) circa September 2021 because it had scale profile wheels that allowed 2-rail DCC operation with a PS3 board. My past experiences converting O Scale 2-rail MTH locomotives are happy ones, and this conversion was no exception. This time, I decided to add a little wrinkle by retaining track-based 2-rail DCC operation, and I’ll show you how I accomplished this.

MTH AC-6 Cab Forward details

Conversion Process for Track or Radio Controlled DCC Operation

The entire conversion centered around the tender – the locomotive was entirely untouched.

I mentioned in the Introduction that I wanted to retain the 2-rail track-powered operation, so after removing the tender shell (which is very easy: remove four screws, and you’re in!) I concentrated on the 2RAIL/3RAIL switch shown diagrammatically and pictorially below:

                        TOP VIEW OF SWITCH
          		     3RAIL
  Center loco roller (Br) ->|    |<- (Bk) Rt tender wheels
          PS3 (Tk R) (Re) <-|    |<- (Gy) Lt tender wheels/tender frame/loco Lt wheels -> (Gy) PS3 (Tk L)
    Rt tender wheels (Bk) ->|    |<- NC
                             2RAIL

2RAIL/3RAIL switch top view inside the tender (original installation)
2RAIL/3RAIL switch top view inside the tender (initial installation)
2RAIL/3RAIL switch bottom view underneath the tender. Note: I changed the “3RAIL” to “RA” (for radio control).

Depending on switch position, the “two-position” switch electrically connects the center row of terminals to either the top (3RAIL) or bottom (2RAIL) row of terminals. The two columns of the switch are electrically isolated; i.e., each terminal in a row electrically isolates from the other.

From this “switchology,” it’s apparent that the “track left” comes from several locations on the center-right terminal, and one wire goes from this terminal to the PS 3 board. The “trick” is to move the several “track left” wires to the bottom-right switch terminal where “NC” was and leave the one grey wire leading from the center-right terminal to the PS3 board in place. Then, disconnect the wires on the top posts (originally for 3RAIL operation) and attach the DCC Track Right/Left outputs from the DCC amplifier that is in turn connected to the ProMini Air receiver that collects RF DCC. Voila!

                        TOP VIEW OF SWITCH
          		      RA (for radio control)
       Radio Control Tk R ->|    |<- Radio Control Tk L
          PS3 (Tk R) (Re) <-|    |-> (Gy) PS3 (Tk L)
    Rt tender wheels (Bk) ->|    |<- (Gy) Lt tender wheels/tender frame/loco Lt wheels
                            2RAIL

Dead-Rail Components Installation

The original tender electronics installation

Several Battery-Powered/Radio Controlled (BPRC or “Dead-Rail”) components required addition: an antenna, a power switch, a charging plug, a 14.8V battery, the ProMini Air receiver, and a DCC amplifier. From the picture above, you can see that the prominent speaker takes up a lot of real estate that we need for our Dead-Rail components. Sadly, the speaker had to go, replaced by a smaller speaker that I mounted in place with 2-sided tape.

The Tenergy 4S1P 14.8V battery has an extended, thin profile that makes the tight fit in the tender. The ProMini Air receiver and the Cytron MD13S DCC amplifier fit nicely on the forward sides of the tender interior.

Tender Dead-Rail installation

Antenna installation was easy, requiring drilling a small hole through the bottom of the tender. The ANT-916-CW-RCS is an excellent choice for a small 915 MHz antenna with 3.3 dBi gain. It is available from Digi-Key or Mouser. 

ANT-916-CW-RCS Antenna installation on the bottom of the tender

I installed the charging plug in an empty hatch opening (with a bit of filing of the original opening) on the top of the tender and repurposed the “DCC/DCS” switch as the Dead-Rail “on/charging” switch.

Charging plug and Dead-Rail on/off switch on the top of the tender
Repurposed switch for Dead-Rail on/charge

Conclusion

Conversion of the O Scale MTH AC-6 Cab Forward was rewarding and straightforward. See the video below for the “proof in the pudding.”

Demonstration of Dead-Rail operation. Note the slight flickering of the lights is an artifact of the video sampling process.