Train from Tangier to Casablanca then to Marrakechm Tangier

Casablanca is at the bottom of the stack. If you want two cities, make it Marakech and Fez.

The only things worth visiting in Casablanca (apart from the airport!) are the mosque and the (faux) "Rick's Cafe" (just to say you've been!).

What about an overnight in Casablanca on the way to Marrakech? Where is the best place to stay in Casablanca?

I agree with Ed and Roger - skip Casablanca. Visit Fez instead, or spend more time in Marrakech, or even add Essaouira if you want the coast. How many days will you be in Morocco?

Agree with others - Casablanca is not really worth a stop. There's one mosque, and it's modern, not historic. It's in an interesting setting, right on the ocean, but I don't think it's worth the time spent to see it. When I was there, infidels like me (non-believers) could only stand outside and crane our necks to try and see what was inside. It may now be possible to go inside, but still, it's a big "meh." Marrakesh is well the trip, and so is Fes. I'd do a day or two at each if you can swing it. Haven't been to Tangier but as the quintessential "bordertown" experience (think Tijuana: is that really Mexico?), would urge you consider skipping Tangier entirely. You know you can fly from Madrid and other cities in Spain to Marrakesh (or Fes) for crazy low prices (under 75 Euro when I did it). You'll spend more getting to the ferry, riding the ferry, warding off the touts at the port, finding your way to the train, taking the train to Casa (where you don't really need to go at all), stay the night there, then finally get on the (surprisingly long) train to Marrakesh. Don't waste your time with all that - just fly direct to Marrakesh for cheap. Consider doing a loop, fly in to Marrakesh, spend a day or two, then take the train to Fes, spend a day or two there. Then push on south if you have time, or fly back to Europe from Fes (again, crazy cheap fares). Hope that helps, and have a nice trip.

David is right on the money!!