New Tube Puzzle

HERE is a carriage serial number from a Northern Line train that makes an interesting Tube Puzzle.
51663
As usual, each number should be used once and once only, and must be added, subtracted, multiplied or divided to make a balanced equation.
I would classify this as of "moderate difficulty."
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My solution: 5 = 3 + 1 + (6 ÷  6)