I did lots of daft things as a kid living near a quiet branch line with a train only every hour at comparatively slow speeds, but I was a nothing more than a näive kid. Thinking back to what I did then, with the knowledge I now have, I realise I was an idiot and I cringe at my stupidity. I thought I knew I was safe, in reality I was just guessing and hoping. I certainly never encouraged anyone to join me (as you are through those videos), I never even risked my dog back then.
The risk of complacency that najaB mentioned is very real, and has caused countless near misses, injuries, and fatalities of people who had worked on the railway for years, and were fully PTS qualified. The moment you begin to feel comfortable trackside is the moment you're most at danger. You should never, ever, forget that there is always potential for a massive lump of metal to come hurtling towards you at up to 100mph, and it will not stop. I've seen too many videos and heard too many stories to forget.
These days even if I'm working with TOWS and lookout protection I'm always ready to move at a moments notice, and aware of my surroundings. On the other hand, you don't realise the risk involved, and aren't aware of your surroundings because you're preoccupied with the camera.
What I'm most concerned about though is the fact you say there's an adult present in your video description. What adult condones messing around on a railway?