Depends on what decoder you have installed - Does it have enough function outputs to drive the ditch lights? Also, what scale and manufacturer are we talking about. And which version of the Tsunami decoder do you have? From a previous thread I see you have an Genesis SD60I - What are the ditch lights doing right now - They can be burned out quite fast with the wrong voltage.
Here is the Soundtraxx link for the '710' engine sound decoder. On page 32 & 33 are the instructions for setting up the ditch lights. You are going to have to verify that the wiring to the ditch lights is correct for the Tsunami decoder, as you replaced the old MRC sound decoder that was in the model.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin