By thinking carefully about how one can measure time, distance, and speed, he showed that they are not contradictory. In the train example, both observers see the beam move at speed c, but time flows at different rates for the two of them. A woman on a skateboard holds a box 1.5 meters tall, with a mirrored top and bottom. Time required is longer than 10 -8 seconds. This result holds for any clocks, not just light beam clocks.
Time passes at different rates for observers moving relative to one another.
If the relative speeds are small compared to c, the differences in time flow are very small. Precise clocks on airplanes flying east run slow compared to clocks on the ground. If they move near the speed of light, they live much longer. Do not agree on the lengths of objects. No object or particle of non-zero mass can be accelerated to the speed of light.