IF you are referring to that "Holy Grail" piece that completes your universe..yes buy a spare. IF it's scarce now..it probably will be later..yes get a spare if you can.
I have seen far too many engine parts available from locomotives that had derailed and launched onto concrete or tile floors. Diecast is not forgiving.
IF your concern is electronics failure, as others suggested, buy some spare compatable aftermarket electronics. ( E,-unit, smoke units, lighting, sound, maybe spare traction tires).
Most shelf Queens, can be restored to conventional use (low cost) or upgraded (market price..usually expensive).
When engines totally fail it is always an unexpected costly loss. There is almost always another of the same model available out there if it's not a limited production piece.
Most of us turnover our motive power as newer models are produced.
( I dont think there will ever be a shortage of NYC Hudsons).
