just use standard 20m rail piece and put it inside both ot them.
you have to be precise, otherwise trains may jump up over parts that sticks out where they shouldnt and end up derailed.
there also is (i think it is in vanilla) a piece in fences that look like train track which you place just like.. fences. it have the same wide as those from mod but its really hard to put them as you would like to.. so use it only as a last resort.