They are called cart ruts because of their resemblance to tracks left by carts.
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There are different theories about how these tracks were created.
The heavy wooden wheels or sleds would have leave furrows on the top soil.
Over time, the furrows would be deeper and deeper until they scrape the limestone floor.
One of the most complicated networks of cart-ruts is found at Misrah Ghar il-Kbir near the Dingli Cliffs.
Not all are of the same origin and not all had to fulfill the same purpose.
Some of them are simply weathered Roman roads.
But the rail tracks on Malta are unique and very impressive.
It is very likely that they are simple traces of transport mediums, such as grinding poles of wood.
Sources:Wikipedia,Angelfire,Saeti
