If you actually watched the sequence in between Ford boarding the bus and the train wreck, you would see that the white guard tosses him the keys so he can help the black guard, who has just been stabbed. It helps to have watched the movie and made sure you know what you are talking about before you post something.
After Harrison's character unlocks his handcuffs he errantly tosses the keys which land within reach of the another prisoner. His expression is half-startled recognition that this guy can now release himself.
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