I'd have to go with a Model L because nearly all of Oregon's drawbridges outside of Portland use either Model A's or L's to warn vehicular traffic. In Portland they have either railroad-style bells or a repeating electronic tone.
I've lived in this area my whole life and never knew there was a siren on this bridge until I started working for the City of Ottawa 2 years ago. I don't know that much about sirens but I thought it was probably a smaller one like a Model A, you can hear it pretty much anywhere in town. I haven't had a chance to get close enough to see it yet. I'm into railroads too, so it's a double treat when I get to see the bridge in action.