What kind of vehicle is in front of the tree?
The vehicle is a train.
    src        edge_attr  dst
0     0   to the left of    3
1     0  to the right of   17
2     0           behind    6
3     1   to the left of    2
4     1  to the right of    8
5     2  to the right of   11
6     2  to the right of    8
7     2  to the right of    1
8     3  to the right of    0
9     3  to the right of   17
10    4  to the right of    7
11    5   to the left of   13
12    5  to the right of   10
13    6      in front of   16
14    6      in front of    0
15    7   to the left of    4
16    7   to the left of    9
17    8   to the left of    2
18    8   to the left of    1
19    9  to the right of    7
20   10   to the left of   13
21   10   to the left of    5
22   11   to the left of    2
23   13  to the right of    5
24   13  to the right of   10
25   16           behind    6
26   17   to the left of    3
27   17   to the left of    0