If a train is going 10km/h and a motor bike is driving on the train at 10km/h. How

20 Km/h, if the bike is going in the same direction as the train. If going in the opposite direction it is doing 0 km/h relative to a person watching (from outside the train).
 
I know you keep asking this question in different ways because you didn't like the answer with relativistic speeds, but both things are true.

If two trains are going 10 km/h, the relative speed is 20 km/hr. The difference between what Newtonian mechanics says and relativity says is too small to measure.

But if both are going 0.6c, the difference between what Newtonian mechanics and relativity say are huge.
 
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