Are the open-air cars available in late December?

No open air cars. All coach type cars. Depending on which class you choose there are chairs in coach. 1st class has more room and comfier.

December 3 trips a day...

No

No they are not available.

I don't know. I also think it would be too cold this time of year.

They have cars available the have windows you can raise or lower. I agree with previous answers though. It would simply be too cold for true open-air cars.

I would not want to ride in the open-air cars in November, December or January either. Way to cold.

Trust me, you would not want to ride in them if they were. Wind chill would be below zero.