Alex,
What I meant to say was I remember it being black when I was a kid. My brother worked for an outfit during the summer that traveled up and down the eastern seaboard cleaning tunnels by using steam or sandblasting when he was in college.
They clean the tunnels periodically to so they can inspect them but in the early 60's that portal was as black as the ace of spades. I guess with clean burning diesels and lighter rail traffic these days it stays clean from the rain.
If you want to know what the outside looked like, look at your pic and look at the ceiling on the inside. That's what it used to look like.
Hope this works Alex this is a pic of the Columbian coming thru the tunnel in 1949
David
