Physics
In a railway track, two rails are joined end to end with a gap in between them because

Accidents due to contraction in winter can be avoided
Air-gaps are necessary for bearing the weight of running train
Accidents due to expansion in summer can be avoided
Steel can be saved

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Physics
In a conductor

There is no conduction band
The forbidden energy gap is very wide
The valence band and the conduction band overlap each other
The forbidden energy gap is very narrow

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