Thousands of people use the metro every day, during rush hour stations are so busy that you can miss a satellite or a child in a split second. If such a situation occurs, do not despair - start looking for the missing person.
There are two possible events here. You fell behind the train, and your companion (child) drove on. To meet again, take the next subway train, which is moving in the same direction as the previous one, and go either to the final station of your trip, or simply to the next station, where you will meet the lost one. This must be discussed in advance.
Your companion (child) was wiped away from the carriage doors by the crowd, you managed to get inside, but he did not. Then take the return train and return to the previous stop. In that case, of course, if the person accompanying you stays in place and waits for your return. And this should also be agreed in advance.
To search for a trapped child, immediately connect the station attendant and the police. Tell us at which station you missed each other, where you were going and what the lost person looks like. Constantly try to get through to the child on your mobile, send him an SMS.
Sometimes it becomes necessary to find a complete stranger who has caught something in you with just one glance. In this case, searches depend only on your persistence. You can visit this station several times at the same time, try to meet the stranger you like (stranger) again.
You can also talk to the metro attendants and, after describing the situation, pushing a pity or paying for services, try to view the recordings of video cameras that are now working at each station, at the exit and at the entrance. At least you will know where your counterpart came from and where it went.
Another way to find a stranger on the subway is numerous sites on the Internet (for example, http://www.diary.ru/~metro-vstrecha or http://s30960825881.mirtesen.ru/) that post wanted ads and thus help you find the person you are interested in.