What is located on top of the bridge?
The tracks are on top of the bridge.
   src        edge_attr  dst
0    0  to the right of    4
1    1  to the right of    2
2    1  to the right of    3
3    2               in    3
4    2   to the left of    1
5    3   to the left of    1
6    4        on top of    5
7    4   to the left of    0
8    4   to the left of    9
9    9  to the right of    4