Vintage Lionel 390 Engine seems to require way too much voltage to run

Were the wheels pressed on too tightly?

Has the commutator plate shifted/rotated from its proper position? If it is a flat commutator plate, compare it to another similar motor and see where the gaps are in relation to the gaps in the armature winding. If they are not clocked properly, the motor will not run properly.

do the wheels move back and forth , there should be some play,  not much.,, armature shaft loose in both bearing plates,, commutator  slots clean,  commuter face,, is it black,  brushes ,,,brush springs.......

field,,,

   Has it been oiled well since the repair? They need that oil, especially any new parts.

  In general it sounds like the brushes and armature may just need another cleaning though.

One of the most common problems with tinplate steam locomotives is if the drive wheels were not quartered properly, or if one or more drive wheels have slipped on the axle, then the side rods will jam.  The side rods should be rattle-loose at all positions; if not, then the drive wheels are using up most of the motor's power trying to work against each other.

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Okay. I did have to do some work on one of the inner side rods. Now they are all loose in all positions. Seems to be slightly better. Motor looks clean and all good. Spoke with repair guy. He told me that the motor was cleaned top to bottom and checks out fine. He also mentioned in his vast experience (and it is vast), that some standard gauge motors just need more voltage. It is moving fairly well at 14 volts now. Thanks all.

14 is reasonable. Give it an hour of run time it should come down some more.

Seems to be getting better. Starts moving at about 10 v and is moving at a good clip by 14.