Cant Remember the name of the movie?

I use to watch this movie when I was little. Dont remember most of it. Its animated. It stars this little boy, who is in this magical kingdom for whatever reason i cannot remember. What I do remember is there is a king who likes trains and wears a conductors costume and has a large(spans the entire castle) train set that you can ride on. Remember the boy gets tutors to learn how to act princely. From book knowledge to fencing. This scene was all in song and as he started to focus on one lesson another teacher would pick him up and carry him off to the next. I remember he opened up a door he wasnt suppose to, and there was this black goop in there. Filling the entire room. It scares him away but he forgets to lock the door. Everyone gets eaten by this black goop. The king had a magic scepter that the boy gets and uses a magic spell to make the goop go away. Anyone know this movie?