adjective
BAD : These new trains are very rapid.
GOOD : These new trains are very fast.
BAD : I rapidly took the gun out of my pocket.
GOOD : I quickly took the gun out of my pocket.
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When used in connection with movement, rapid and rapidly usually refer to something that happens at a very fast rate: 'She could hear rapid footsteps approaching.' 'The patient often develops a fever and breathing becomes rapid.'
Note that these words are mainly used to describe the speed with which something changes, develops, spreads etc: 'Investors have profited from the country's rapid economic growth.' 'Unemployment has been increasing rapidly over the past 8 years.'