The piece of evidence that best supports the claim that gravitational potential energy was converted into kinetic energy as the train rushed down the hill is the increase in the train's speed as it descends the hill.
As the train moves downhill, its potential energy decreases due to the decrease in height. This potential energy is transformed into kinetic energy, which is the energy of motion. The increase in the train's speed as it moves downhill is a clear indication that its potential energy is being converted into kinetic energy. The faster the train moves, the more kinetic energy it possesses.
Observing the change in speed and the corresponding increase in kinetic energy provides direct evidence of the conversion of gravitational potential energy into kinetic energy, supporting the claim made in the statement.