Which kind of vehicle is gray?
The vehicle is a train.
    src        edge_attr  dst
0     0           behind    4
1     1  to the right of    5
2     3               on    4
3     3   to the left of    6
4     4             near   10
5     4      in front of    0
6     5               of    4
7     5   to the left of    1
8     6  to the right of    3
9     6               of    4
10    8      in front of   11
11    9               on    8
12   10          next to    4
13   11           behind    8