Lionel 601 question re motor rotor

It should have both a metal washer and a fiber one on top of the armature.

Sounds like you're in urgent need of some fixemup. Even though the service drawing for that motor doesn't show two washers, I completely agree with Chuck. Both washers mount on top of the armature - the bottom washer should be a 600-129 nylon thrust collar, while the one directly above it should be a CON-8 metal thrust washer. Both parts are available from Train Tender, but I'm certain there are many others. I just find Jeff to be a top notch parts dealer. At $0.25 each, I would certainly order at least two of each, and see if one nylon washer does the job or if you have a better fit with two nylon thrust washers - either way, be sure to use the metal washer above it - the metal washer is quite thin, but it provides a smoother surface between the brush plate and the nylon washer. While you have the armature out, be certain to take a peek down in the plastic bottom plate to make sure you still have the 600-110 bearing ball in place. If missing, that would allow the armature to drop too low, thus creating extra play during direction changes. 

The goal is to have minimum vertical movement of the armature when changing directions. The three armature poles should be centered within the field laminations for max power and efficiency. While I'm not aware of any spec, I would estimate 1mm-2mm max vertical movement would be right. 

May as well clean the commutator and replace the brushes while you're in there too - and replace both springs if they feel really weak or if they don't match in terms of tension (they're cheap too!)

Jeff also has a bulk list with fairly low minimums. Both of those washers are cheaper by the dozen. That is how I buy them.

I would also check to see if the top plate is coming loose on the sides of the wheel frame.