Postwar Lionel NW-2 only runs one direction

this is not normal, so there is no normal cause 

The first thing I would check is the e-unit. I'm not an expert on fixing these but if you do a Google search there are some great you tube vides on how to fix, clean and repair them.

The "normal" cause of this is that the E unit is locked in one position. Sometimes the E-unit lever works its way to the "lock" position in the older units.  Renovo_PRR is correct, the E-unit may need servicing.

I just worked on an engine where the other finger in the e-unit was not touching the drum, so it has no reverse. That would be my thought if your e-unit isn't locked out.

If an e-unit is locked, then the drum doesn't cycle so it stay in on mode.  So if it runs in forward it stays in forward, no neutral.  Since his goes in the sequence, just misses reverse it is powered correctly.

It easily can be an e-unit finger or drum contact worn/dirty.  Finger are thin and can be consumed by the sparking/corrosion, or bent.

Since it runs in forward the armature should be good.  When it hit the reverse spot does the motor hum or try to move?  If so maybe the armature jams if the bearing on the motor are too worn.  I have seen this occasionally but mostly on later MPC stuff.  G

Chris,

I like your jewelry!

My post war E-unit had a hardly noticeable slight bend of the contact finger, being away from the drum.  How it got that way, could have been when I cleaned the motor and E-unit and me NOT being CAREFUL!

Be very careful, as these contact fingers are very delicate.  I broke one off and had to buy a replacement!!!!

Ralph