Question 1132127
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The rails are what the train's wheels ride on.  The cross pieces are ties, not rails.<br>
5km = 5000m = 5000 ties.<br>
That is, 5000 ties after the one at the beginning of the 5km track.<br>
If the dog starts at 1m/sec and increases by 1m/sec every meter for 5000 meters, his speed at the end of the 5000m will be 5001 m/sec.