In a train we get in and find our seat, get introduced to people around, develop some sort of impressions and help each other if needed being co-traveler. When our destination comes we get down from the train and do not remember any one after some time.
Similar situation is here in this life. We get birth get the education from our teachers and parents, do some kind of job and when the time comes depart to the destination.
As we do not get attached to any one in a train journey, as we know that this is just a small time period we are together. Same approach should be followed in the life. This life is also a small period if we look at the total time period of billion of years the creation came into existence.
Thus as we behave in the train journey, that is just having casual a purposeful relationship with other travelers in our cabin or coach, same approach should be followed with people around us, in this world. We should have limited relationship and emotional bonding in this world as we do have in the train journey.