It's full of ice cream and frozen Christmas cakes.
6 seconds sound clip from the Frosty the Snowman (1969) movie soundboard.
You can hear this line at 00:12:26 in the Blu-ray version of the movie.
Quote context
[...]
- Now I'll never get to the north pole.
- Oh, Frosty, you just can't melt!
- Oh, Karen, don't you get all slushy too.
- What is it, Hocus? Out the window?
- A refrigerated boxcar on a train headed north. You'll be safe there, Frosty! Come on!
- It's full of ice cream and frozen Christmas cakes.
- Hurry up, Frosty, the train is pulling out!
- Are you coming to the north pole, too?
- I'm sure my mother won't mind, as long as I'm home in time for supper.
[...]