Nice YouTube, Jman.
Why do the bridges have a roof?
And why the windows? It's not like you are going to stop and look out the window at the view
I think that the roof was to keep snow and rain off the trusses and the planking and make the bridges more resistant to rotting. Many of these bridges are over 150 years old.
My guess is that the windows add natural light.
The original paint was red like the paint on barns because paint helped preserve the wood and it was the only paint color available (from a homemade mixture of iron oxide, lime, skimmed milk and linseed oil).