Lionel Postwar 1666 X

That's difficult to say why the X on the cab of an engine.

E after an engine number is more common but pre war thing.

I have found the elusive X listed for several post war freight cars, mainly Pennsylvania and NYC.

I have looked in the Greenberg's pocket price guide, in the front and rear sections, that I have and no explanation is given for the X letter.

Lee Fritz

Wes

the only explanation of the X that I have found is that they may have been part of a set. 

I purchased a PW box car that was  numbered with an X.

Looking in the Choo Choo Auctions price guide, the sets were what I was able to find a X marked pieces.

http://www.traincity.com/price_guide/pllistp.html

Dave