I saw a placemat in a market which was made of a leather imitation. This looks a lot like a cobblestone pavement.
The placemat was already a very light gray, and I left with a mixture of water, a drop of washing-up liquid and a few drowning black paint. Simply "smear" over a large area with a large brush. That then digs nicely into the small joints.
I then wiped the still wet paint off with a sponge.
When the paint had set, so it was so slightly sticky, I aged the whole thing with a little black and brown patina powder.
In order to let the tracks into the pavement, I cut a thick cardboard box to match the track geometry. Then cut out the same shape from the tsichset and glue both on. For the strip in the middle I just cut off a strip that is a little narrower than the clear dimension between the rails. there is also a thin cardboard strip underneath to get to the same level.