Yes, I do understand you now. I believe the siren doesn't always work properly so you can't do that. And changing trucks in the station would require a lot of skills. It's not common to do that in this mod. I'd just leave it as it is.
oh...so its hard, right?! So sad because there is a little bit fire trucks at the station...just 2 fire truck, 1 ladder, 1 rescue and 1 transportation....