You could just switch the speaker. But as the MLS's power input connections are wired parallel to those from the incoming track/ESC voltage and battery/back up, you could swich the whole card out, not just speakers. Are you track or battery operation ? You could even install multiple switchable MLS cards in the trailing car

Have a sound for everything on your roster ! They make diesel, petrol railcar and tram. I have all 4 types. Make them switchable depending on what is pulling the trailing car. Max.