If you are unfamiliar with U-Bahn, here is a guide that covers the core logic.
Draw a loop network through the centers of some cells, which may branch or turn, but may not have any dead ends.
In the rectangular areas outside the grid on the top and left, a clue indicates how many times the corresponding line shape (i.e. a cross, branch, straight line, or turn) appears in the corresponding row or column, irrespective of the line shape's rotation.
Assign a value to each line shape:
turn = 1
straight line = 2
branch = 3
cross = 4
A clue directly outside the grid is a Skyscrapers clue, which indicates how many cells in the corresponding row or column contain a larger value than all cells before it from the direction of the clue.