Question

A clock on a moving train runs __________ an identical clock at rest.

Answer 1

It depends who is watching it.

-- A passenger on the train says that his clock is normal, but the clock at the station he just passed is running slow.

-- A ticket agent at the station says that his clock is normal, but the clock on the train that just rolled through is running slow.

-- They're both correct, but ...

-- The difference is too small for either of them to notice, unless the train is going faster than about half the speed of light.

-- It isn't.