Located 12 miles from Three Crossings Station and 12 miles from Warm Springs Station just north of the Old Emigrant Road and South of Ice Springs Creek.  http://www.expeditionutah.com/featured-trails/pony-express-trail/wyoming-pony-express-stations/

Bishop and Henderson, as well as the mail contract of 1861, identify Ice Slough or Ice Springs as a C.O.C. & P.P. Express Co. station between Three Crossings and Warm Springs.  Other sources do not list the slough as a station, but they mention its nearly year-round ice as a well-known landmark.  Franzwa locates Ice Spring and Ice Slough separately in his Oregon Trail maps, but he does not list either site as a Pony Express station.  http://www.nps.gov/parkhistory/online_books/poex/hrs/hrs6a.htm#77

This is possibly the station Twain mentions as passing through in the night. He tells of buried ice there that can be carved out for easy use.