Chicken: youthful slang. Someone is ‘chicken’ if they are afraid to do something.
screech: the squealing noise that car brakes make when a car stops suddenly.
they questioned him: they asked him a lot of questions in order to find out what really happened.
he needn't have worried: there was no reason for him to worry.
the rush hour: the time, morning, and evening, when everyone is travelling to or from work.
Beware of: used in notices to warn people of a danger, for example ‘Beware of the dog’
pickpocket: someone who steals things from people's pockets.
He worked his way over: the platform was crowded so it was not easy to get to where the tall man was standing.
bumping into: knocked against. It also means ‘to meet by chance’: ‘I bumped into an old friend the other day’
slipped away: left, hoping that nobody would see him leaving.
grabbed: got hold of him quickly before he could get away.
dragged him: the pickpocket didn't want to go so he had to be dragged - pulled along.
check: have a look to make sure of something.
his childhood friend: a friend he had when he was a child.
rob: if you rob a person or a bank, you steal (something) from a person, a bank, etc.