This behaviour is not at all uncommon no matter how many bad-tempered posters try to suggest that it is. Many teenagers go through phases of behaving in an antisocial manner. Impulsivity is part of the process as the teenage brain struggles to cope with all the changes going on. I highly recommend reading up on the teenage brain, it can be very enlightening.
Yes, the shopping trolley could have landed on someone and hurt them, yes he could have been badly hurt on the train tracks. But let's not pretend he is a delinquent through and through, and that this sort of behaviour is extreme; it's not.
It is worrying though exactly because it can be so dangerous. Oh to be able to protect teenagers from themselves.
Like a previous poster, I wondered if your son might be dealing with ADHD as well?
In all likelihood he is feeling deeply ashamed and full of self loathing. It is great that he has such caring and devoted parents, he really needs you right now.
I think your idea of part time work is a really good one. I've heard of quite a few young people with ADHD (not that your son necessarily has this) transforming attitudes and behaviour upon entering the workforce. Is he good with IT? Might he be interested in or eligible for an apprenticeship as an electrician, mechanic or engineer? If he is earning, learning and doing something productive in a more grown up environment, he may be able to shake off a lot of the immaturity.
Do be warned though that the transition to the workplace is huge for young people and they tend to find the first six months extremely tiring. So he may need to focus on just work for a few months and even then, just 20-25 hrs.
Keep in touch with the GP, keep talking to supportive friends, keep assuring him he is loved and cherished, keep guiding him by example and ensuring he knows he can talk to you even when he has messed up, and assure him that although he's lost this chance with the school that there are still many possibilities out there for him and you will support him to achieve.
He might benefit from some very specific-type therapy like breaking down a behaviour and exploring ways that might work better. Younger children do this through cartoon-like pictures. A way of helping them to imagine better outcomes, of linking behaviour and consequence.
In the end, what matters most is his health and wellbeing, and that he has a family that loves him. The marks and stuff are neither here nor there.