Hogwarts Express derailing

As to constant speed, that is relatively normal for ready to run sets from lionel.  They will normally slow down in the curves as they do not have the fancy electronics to make them go a constant speed.  Without seeing a video or example of what your saying, my assumption is that the speed reactions are normal.

derailing is not normal.  I have a hogwarts set and it does not derail.  There was a video about the front 4 wheels (pilot truck) being un-even on the rails and then derailing.  You just needed to bend them down.  hopefully someone has the video link to share (I don't have it handy or in a position to search for at the moment).

The Hogwarts engine requires O36 curves.  I heard that some sets were issued with O31, not sure if this is true, but check the radius of your track.

Bill

The Hogwart's Express will NOT run on O31 if that's what you have.  Been there, tried that, burned the T-shirt.

How do I tell the difference between the O31 and O36 track?

Measure the diameter of the circle of track. What should have come with the set is 36" dia. Fastrack. 

Another thing to check: make sure any cardboard or foam pieces from the packing have been removed from around / under the wheels - especially the pilot truck - I've missed this myself on new locomotives, so take a look. 

It's a good set, so I hope your kids enjoy it. Merry Christmas !

Tell us what brand of track you have to start.  If it's Lionel Fastrack, every curve has the diameter molded into the bottom of the roadbed.

Track is 36 fast track. How mobile should the 4 little wheels in the front be?

my uneducated guess is that there's something too loose about them- it's almost like when it goes around the turn, that part is bumping into the front big wheels. Is this a common thing and is there a quick fix?

You're going to have to post some pictures. 

When I tried to run the Hogwart's Express on O31, that's what happened.  However, it ran fine on O36 Fastrack, and that's what comes with the set as well.

From what you describe of the little wheels (pilot) hitting the big wheels (Drivers), it could be that the forward (pilot) truck (assembly with four little wheels) is reversed, and the pilot should be rotated 180 degrees, so that the little wheels that are hitting become the leading or forward most wheels.  These truck assemblies are often symmetrical, but sometimes not.

Bill

https://www.youtube.com/watch?v=ceXfQL3p35M

Good luck - it's a beauty when it runs.