Choum is a town in Mauritania. Practically impossible to hitch to/out of here from Atar - the town only exists because the train stops here, and the only vehicles going in and out will be bush taxis coming to meet the train or taking passengers away to Atar. These can cost between 1000 and 2000 ouguiya depending on how many passengers are waiting and your bargaining skills.
There are 3 trains a day to Nouadhibou and 3 to Zouerate. Local people will tell you when and you can have a tea and good fun with them while waiting. First train to NDB should leave at 22:00 but it came at 23:50 and took exactly 12 hours. It's full of iron ore, prepare sleeping bag and close in it. Otherwise you'll have black dust everywhere.
Like in Nouadhibou, be careful of straying north of the tracks. Choum is right on the border with Western Sahara and this border has been heavily landmined.