You may already be aware of the exploded diagram: https://www.lionelsupport.com/...amp;CFTOKEN=33055634
Taking a few shots in the dark:
- The plunger - is it bent? Replace it. Is it magnetized? You can replace it or use a screwdriver [de]magnetizer to clear it up. Run it on AC and not DC to prevent recurrance.
- Coil partially shorted/weak - you could replace or re-wind the coil. The hard part is being careful not to break the metal tabs off of the 455-26 coil retention bracket. Some swear that heating the tabs with a soldering iron before bending lessens the risk.
- Groove Pin Binding - lubricate the 455-57 pin so that the beam/arm moves freely. Also ensure the pin is not bent.