Everything happens for a reason. If you’re raised to respect authority, hard work and honesty, you will more than likely grow up to be a well-behaved, hardworking and honest person. If you’re brought up without being taught those lessons, then you won’t see anything wrong with destroying something for the fun of it.
Some old train probably doesn’t matter to the person who did this. I’m not yet 16 but being a teenager is difficult. There’s all sorts of stuff to navigate and if you haven’t been taught these things about how to behave and how to be respectful, etc. then, again, the thought process is probably “what’s wrong with breaking into that carriage, it’s not yours, it doesn’t affect you, it’d be good craic, the lads will call you a pussy if you don’t do it, sure it’s only a heritage railway they don’t have proper fences, it’s dead easy to get in”, etc. etc. etc. Don’t underestimate how upbringing affects behaviour. The environment in which you develop and in which you live has everything to do with how you behave.