What is the name of the vehicle that is gray?
The vehicle is a train.
    src        edge_attr  dst
0     0   to the left of    6
1     0               in    2
2     0  to the right of   11
3     0   to the left of    3
4     0   to the left of    4
5     1               of    0
6     1   to the left of    7
7     1   to the left of    4
8     3  to the right of    0
9     4           beside    0
10    4               on    3
11    4  to the right of    7
12    4  to the right of    0
13    4  to the right of    1
14    6  to the right of    0
15    6  to the right of    8
16    7   to the left of    4
17    7  to the right of    1
18    8   to the left of    6
19   11   to the left of    0