That's not a stone deck over the rails. Just a line of heavy stone along each edge of the rail deck. The deck is solid rails laid side to side, with ballast over the top. The stone sides are to retain the ballast.
This culvert is over one branch of Rapid Creek. I've examined numerous rail-topped culverts of the BCR&N, topped with old 65# rail built on Cedar Valley Quarry limestone blocks. This is the only such culvert I've examined that has 2 spans with an intermediate pier, and the only one I've examined that was built under what must have been 2 or 3 parallel tracks, and the only one I've examined that is on a diagonal to the centerline of the former track. In addition, it is the only one I've examined with two layers of rail. One layer is perependicular to the axis of the culvert, the other is parallel to the line of the track.
Thanks Andrew for getting out to this! It's an example of a structure the BCRN was fond of building that's a stringer made up of old rails with a stone deck.
Thanks, I've never seen one with the "deck" missing.