What is it about kids and trains that defeats all laws of probability - the chance of finding a crying kid in the same train as oneself is always 100 percent!
Crying kids have a knack of shouting at the highest possible pitch, for the longest possible duration and still maintain their sweet innocence. This is certainly not advised for adults though.
A kid's crying has a great social networking effect as well - there is always some other kid that takes its cue from the crying kid and starts off on another track. When they criss-cross each other's path during their crying, various ragas and alapanas can be deciphered.
There is also kumbakarna's third law of sleep intervention that applies here - a crying child will momentarily stop crying to give you a slight belief that you can finally sleep. But right after you do start your nap, the crying is louder than ever before.
Parents of course try their best but they typically add to the kid's reasons for crying! So next time, you should never try to frighten a kid to stop it from crying. Remember the sight of a crying child can frighten a sleepy you much more!