First we start from the definition of synchronization itself. This is known as the rate of change in the occurrence of an event to occur at the same time. If the light clocks of a train are verified it will be possible to observe that there are different verification times between each of them different. This is because the light of the clocks at each end of the train does not appear instantaneously over them, but travels from one direction to another; the light at the rear must have traveled a distance that will cause the first clock to have changed position.
In this way, if a clock of these is observed at the same time, the light will generate that the first one is out of sync with the last one, which would cause the clocks not to be synchronized.