What vehicle is gray?
The vehicle is a train car.
    src        edge_attr  dst
0     0   to the left of   15
1     0   to the left of    5
2     1  to the right of    2
3     1  to the right of   11
4     1   to the left of    3
5     1  to the right of   10
6     2   to the left of   11
7     2  to the right of    4
8     2   to the left of   13
9     2   to the left of   10
10    2   to the left of   14
11    2   to the left of    1
12    3  to the right of   10
13    3  to the right of   11
14    3  to the right of    1
15    4   to the left of    2
16    4   to the left of   11
17    4   to the left of   10
18    4   to the left of   13
19    4   to the left of   14
20    5   to the left of   15
21    5  to the right of    0
22    5  to the right of    6
23    6   to the left of    5
24    6   to the left of   15
25    7  to the right of   19
26    7   to the left of    8
27    8  to the right of    7
28    9  to the right of   21
29   10   to the left of    3
30   10  to the right of    4
31   10   to the left of    1
32   10  to the right of    2
33   11  to the right of    4
34   11   to the left of    3
35   11  to the right of    2
36   11   to the left of    1
37   13  to the right of    2
38   13  to the right of    4
39   14  to the right of    2
40   14  to the right of    4
41   15  to the right of    0
42   15  to the right of    6
43   15  to the right of    5
44   15  to the right of   20
45   19   to the left of    7
46   20   to the left of   15
47   21   to the left of    9