Arnature heats up

It is possible the brush springs have weakened or the brushes are not free to move and therefore the brushes are unable to keep good proper contact with the armature.

Which brushes are you using?

It's well documented here that some recent Lionel brushes have very high resistance and shouldn't be used in a postwar locomotive.  Try to source some original brushes from an original postwar piece and try again.

ATG, you need to move this to the 027 forum. 

For each brush part number I divided them into three groups. There is the postwar copper plated brushes. Then there is the postwar unplated brush, which has a very high copper content.  And then there is the modern brushes, which are mostly graphite. The copper plated brushes are getting hard to find. There seems to still be a lot of the high copper content brushes out there.  These two are the only brushes I use in my postwar trains. The plated brushes are readily appearent by ther appearance. The high copper content, I think they are nearly 80% copper, are black with copper specks and are much heaver than the graphite brushes, which are gray to silver in color with no hint of copper. The most notable thing about the newer brushes is how light weight they are.  None of them weigh very much, but when handling them it quickly became appearent how much lighter the newer brushes are. Forum member CW measured brush resistance and found that new Lionel brushes, ones with shunts on them, had a resistance of ten times what postwar brushes were.  He was trying to get answers as to if this was by design, or if these were just defective brushes.   As far as I know, he never got an answer. But the bottom line, for me, is use postwar brushes in post war equipment. 

Brush resistance is an important design parameter in motor design. The designer wants power to get into the motor, but wants to limit the current flow from bar to bar when the brush crosses from one commutator bar to the next. On larger motors, where there is more room to work with, the brushes are usually long and thin running the length of the slot. They are typically split increasing the bar to bar resistance without increases the shunt to bar resistance. But Lionel just used their little round brushes and let the brushes spark as the brush passes over the slots.  This results in the more frequent brush changes and more commutator maintenance.  But given that these brushes are used in toys, I think they did well.

if you use the search function you might find CW's old thread on brushes   It had a lot of good information in it