on board vs. a board

[Majid72]

Hello

Could someone tell me whether there is any difference between on board and a board(both meaning getting on a ship,plane,train)?

Can they be interchangeably used in all occasions?

Thank you

[Kitosdad]

Yes they can. On board, and aboard mean the same same.
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[Berlinder]

Dear Mr Kitostad,
'on board' and 'aboard' is not always the same,
and definitely not same same.
'aboard' has more than one meaning, depending upon sequence or circumstance.
'aboard' = on board, overboard, to throw sth. overboard; (Dictionary).
Hence the famous phrase: "Man overboard!" when someone has fallen from the ship into the water.