Bob Liberman, an avid SP modeler shows the correct Soundtraxx Tsunamis to use for the different Cab-Forward configurations.

Choosing the Appropriate Tsunami for your Southern Pacific Cab-Forward*
Two Different Air Pump Configurations produced Noticeably Different Sounds!

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The Soundtraxx Tsunami decoder 826105 is made specifically for the Southern Pacific Cab-Forwards that have their air pumps located on the back end of the boiler between the engine and tender.

The Broadway Limited AC-4&5 Cab-Forward have the air pumps located on the right side of the locomotive and the air pump exhaust pipes run through the exhaust stack. Use the Soundtraxx 826103 Heavy Steam.

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Picture one shows the second Cab-Forward with air pump stacks that went directly to the outside to the air and were not muffled by the boiler. Picture two is a close up of the exhaust pipes from the air pumps.

AC-4 4100 to 4109 ----------- Use the Tsunami Heavy Steam Decoder, 826103
AC-5 4110 to 4125 ----------- Use the Tsunami Heavy Steam Decoder, 826103
AC-6 4126 to 4150 ----------- Use the Tsunami Cab-Forward Decoder (no air horn), 826105
AC-7 4161 to 4176 ----------- Use the Tsunami Cab-Forward Decoder (no air horn), 826105
AC-8 4177 to 4204 ----------- Use the Tsunami Cab-Forward Decoder (air horn), 826105
AC-9 3800 to 3811 ----------- Use the Tsunami Cab-Forward Decoder (air horn), 826105
AC-10 4205 to 4244---------- Use the Tsunami Cab-Forward Decoder (air horn), 826105
AC-11 4245 to 4274 --------- Use the Tsunami Cab-Forward Decoder (air horn), 826105
AC-12 4275 to 4294 --------- Use the Tsunami Cab-Forward Decoder (air horn), 826105

All are Cab-Forwards except the AC-9 which was built as a conventional engine for the Rio Grand Division and were built as coal burners and converted to oil in 1950.

The AC-6 was the first engine to have the air pumps on the boiler back. This locomotive was the same configuration as the AC- 4&5.

*Courtesy of Bob Liberman