What is the vehicle that the sign is in front of called?
The vehicle is a train.
    src        edge_attr  dst
0     0  to the right of    6
1     0  to the right of    3
2     2           behind    5
3     3   to the left of    0
4     3   to the left of    6
5     3   to the left of    5
6     5  to the right of    3
7     5      in front of    2
8     6  to the right of    3
9     6   to the left of    0
10    6   on the side of    2