What is the speed of the train?

Question

Two cities A and B are 1200 km apart on earth. A passenger sitting in a super-train that is moving on a straight road passing the two cities claims that the distance between A and B is 612 km. What is the speed of the train?

Answer 1

I think that the question is incomplete.You should have mentioned something about the time too.

Answer 2

this is relativitic time dilation, as the passenger must be using a stopwatch to measure the distance between the cities. I'll assume he somehow knows the velocity.

t'/t = √(1-v²/c²)

612/1200 = √(1-v²/c²)

0.2601 = 1 - v²/c²

v²/c² = 0.7399

v² = 0.7399c²

v = 0.8602c

so the speed is 86% of c or 2.58*10⁸ m/s