I don't know anything about the Sierra cards as I used Phoenix. On Phoenix, the sound triggers just needed a ground to activate. Railpro LM lighting outputs are a switched ground. SO, if Sierra works the same as Pheonix, you would use a Light, not an action. Actions are built-in functions like Load and Brake.
You may want to include a diode in the line just to protect the LM from any stray voltages on the trigger input.