Final answer:
Time dilation is a relativistic effect where time passes slower for an observer in motion relative to another observer. At high speeds close to the speed of light, this can lead to observable differences in time and length measurements by different observers. However, for the speeds at which regular trains travel, these relativistic effects are imperceptible.
Step-by-step explanation:
Understanding Time Dilation and Motion
The phenomenon that you are asking about is known as time dilation, which is a concept from the theory of relativity. Time dilation occurs when an observer in motion relative to another observer measures time to be passing more slowly. Although this effect is pronounced at speeds close to the speed of light, it is imperceptible at conventional speeds like those of a train. According to the theory, the person on the train and an observer on the ground would both agree on the relative speed of the train, but they may measure different durations for the train's journey. This is due to the fact that speed is measured as distance divided by time, and if time is dilated, then the distance can seem shorter for the person moving at relativistic speeds.
So, when a train is in motion, to an observer on the train, the train's own length does not change; however, to an outside observer not moving with the train, and especially if the train could somehow achieve relativistic speeds, they could measure the train’s length as being shorter due to an effect called length contraction. This effect, in combination with time dilation, ensures that the speed of the train (distance covered per unit of time) can be agreed upon by both observers.
In your specific case, the discrepancy in time experienced by passengers versus stationary observers is negligible because the speeds are too low to result in noticeable time dilation or length contraction. At the speeds most trains travel, both observers would measure the same time for the train to cover a certain distance, and the laws of relativity would not need to be applied to explain their measurements.