Anyone know how to get there? repconn site won't let me go south and all the other places don't seem to work.

just found it by mistake nvm.

Go to http://fallout.wikia.com/wiki/Ranger_Station_Charlie then go south-west.

Next to the railroad tunnel entrance south of ranger station charlie.

Get the explorer perk. It shows it to you on the map.

Also, yes, you could just go up the mountain a bit from the railroad tunnel entrance south of Ranger Station Charlie.

Happy hunting!