So this weekend I finally got to see these markers for myself, and my interest is definitely piqued. I didn't spot all that many markers, just a couple of little purple flags and a couple of stakes with orange ribbon on them. One of the stakes is right by the left hand side of the tracks so close it's almost within reach of the passengers, and the other is in the tree line on the far side of the cabin from TGS. The purple flag I saw is also in that tree line, and I saw the remnants of another purple flag near the tracks. They're very hard to spot, so I didn't get any photos. I assume most of the markers are down in the valley or farther back behind the cabin where we can't see. Once the train comes out of the tunnel you've only got a few seconds to look back and to the right to see them, and you have to look close.
Anyway, the purple flags are usually used to indicate reclaimed water lines. I don't know what that could mean, but maybe they have a culvert or something close underground that needs to be moved. If they were pink, that would mean they were just temporary survey markings, but I think these were definitely purple.
The stakes however are obviously more permanent, which I would like to think points to something larger going on here than just working on a water line. I would think you'd only drive a stake marker if you've got a big project where your markings have to stay resilient for a long time...
It'll be interesting to see if they use this off season to do anything like move power/water utilities around this area in preparation for whatever is coming.