Be prepared for an adventure!
We did this about 12 months ago without prebooking. Next time I'd try and prebook, but we got on - even though we ended up paying through the nose to hire an extra flat-bed carrage on the train (about USD 200 IIRC).
Take 4 good quality tie downs (like the not-cheap heavy duty TRW ones that Demon Tweeks sell in the UK), you'll not regret spending the money when you see how they fasten your vehicle onto the train with bent over 6 inch nails and some cable.
We were trying to get to Choum. We did stop there, but we couldn't get off. We ended up going to Zouerat, and were shunted about for hours. In total, we were on the train for about 24 hours, but surprisingly we slept well overnight - keep plenty food and drink handy. It did get cold sitting in the landrover - we had the heater on a couple of times overnight and also I've never been so dusty in my life! Still, it beats driving or spending more time than necessary in Nouadhibou...
If you get off at Zouerat, be careful! They drive on the left, just like the UK, due to the quarry operations there. It was handy having a right-hand drive car while it lasted!