I hope this isn't too frivilious of a question. But I've seen these horizontal slats or boards placed at intervals on several older land bridging trestles. I'm curious, what was their purpose?

I speculate that the partitions were made of lumber specially treated with a flame retarding chemical, and were installed at intervals as firebreaks. The intent might have been to divide the trestle into sections, and avoid burning down the whole bridge in the event of a fire.
Dan Rohrback