According to the theory of special relativity, an object with mass cannot reach or exceed the speed of light in a vacuum. As an object with mass approaches the speed of light, its relativistic mass increases, requiring more and more energy to accelerate it further. Therefore, it is not physically possible for a train with mass to reach the speed of light.
However, let's consider a hypothetical scenario where the train is traveling at a speed very close to the speed of light relative to an observer outside. In this case, time dilation would occur, and the clock on the train would indeed appear to be slower to the observer outside the train.
From the perspective of the observer outside the train, due to the high relative velocity, time on the train appears to be passing more slowly compared to the observer's own reference frame. This means that the clock on the train would be observed to tick slower by the external observer.
It's important to note that this scenario is purely hypothetical since no massive object can actually travel at the speed of light. However, the concept of time dilation at high velocities has been experimentally confirmed through various experiments involving fast-moving particles, such as muons in particle accelerators.