Every day, at the start of each hour, a train leaves Ouagadougou and travels toward Nouakchott.
The journey takes ten hours. If a train leaves Nouakchott at 1:30 AM and begins a ten hour return
trip to Ouagadougou, how many trains will it pass going the other way (toward Ouagadougou)?

I do the train trip Antwerp - Paris every week. It takes 2h10min.
The train leaves at 5 min's before every hour and arrives at 5 mins past the hour.
The train back leaves in Paris 5 mins before the hour. (symmetrical to the hour).
I can set my watch that at every hour, and every half hour we pass a train in the other direction. That is 4 in total.

So I bet: 20? Starting to cross the first at 1.45 am, + 19 more the last at 11.15 am.