derailing locomotive

SJC may be right.  How fast is "not going very slow"?

 

The 2001 Pennsylvania Flyer set came with the ubiquitous Lionel 2-4-2 which is not known for being particularly troublesome.

 

If the problem does occur at really low speeds (as low as you can make it go) and is always the front truck the things to check are first: a) is the track level side to side and not changing in slope too much in the curves where it de-rails?, and b) has the front truck come loose or been restricted in movement in some way - your loco is 11 years old and eleven years of play could have loosened or weakened something. 

 

Does it always de-rail at exactly the same place? Even if you put it back on in a different place and run it slow?   If so, then it is a track problem most likely.

I have never had a problem with the many 2-4-2s and 4-4-2s on o-27 curves but if they are going to fat in my experience if they are going to fast they will flip over,

Ck the front truck, sometimes it gets spun around (reversed) on its pivot and will easily derail. Joe

well with the speed control that came with it is about half way in the green and it looks to be leaning alot to the outside.  and wwhat way do i flip the switch to turn off the stack.i flipped the front truck around so the long side is now to the front

About half way up on conventional might be about as fast as some locos will go around an 0-27 curve.  I don't have exactly that loco but I have some that will run off the rails at around that voltage - in fact I have one old (post-War) 2-4-2 that will flip off on O-36 at much over half voltage. 

 

Does it literally tump over and all off the rails? Or does it stay upright and mostly on top of the rails but with the front truck misalligned and off its rails?

Sometimes single axle pilot trucks can be troublesome. You could try adding a little weight. I taped two nickels on top of a pilot truck, to stop it from derailing.  Also, add a little grease if the truck doesn't swivel easily.

CandO is right about a few coins helping keep a one-axle (or even a two-axle) pilot truck on the rails.  You might try this.   I've used up to four quarters on one troublesome pilot.  No idea whey it needed the added weight but it dixes it so you just move on.

 

usually the first axle of the truck pops off its a 4-4-2 .im guessing when it does flip its going to fast

The leading trucks of my GG1 derails on 072 curves.  Problem solved by banking the outside rail with small (1/8 ) shims under the outside rails.  Very little effort to place shims under the ties.  This may help your current situation. 

HO car weights on the shaft connceting the front truck might work!