Thanks for the reminder to go over our family „rules“ for how to deal with this, again.
Namely:
If a child gets left on a platform, the instruction is „don‘t panic, but STAY EXACTLY WHERE YOU ARE. The adults will turn around at the next possible opportunity and return for you.“
If a child boards a vehicle and we don’t, „remain calm, and GET OFF AT THE NEXT STOP, THEN STAY EXACTLY WHERE YOU ARE. We will be along to collect“.
If the child can see someone like public transport personnel, or police, tell them what happened, and explain that you are waiting here for your parents, your family has rules for this, and your parents will return for you as soon as they can. You can ask them if you can use a cell phone to call us - speak to me yourself, don’t simply believe them if they spoke to me. (all of the kids knew my number off by heart from about age two - if yours don’t know your number yet, then my top tip is, give it a daft catchy tune, make a kind of jingle out of it and sing it together every morning at breakfast).
Other possible sources of help include parents with children of their own. Go by your gut feeling, if it feels wrong, then listen to that feeling.
In any case, refuse to go off with anyone, anywhere, no matter what they tell you.
We discuss this scenario regularly with the kids. It started because my oldest got in a flap one day about 3 years old when he realised it was possible to be left behind. Having the rules helps to keep everyone calm and safe.