What is under the trains in front of the trees?
The gravel is under the trains.
    src        edge_attr  dst
0     1      in front of   14
1     1            above   10
2     1            above    5
3     1            above    7
4     1      in front of    4
5     2  to the right of    6
6     2  to the right of    3
7     3   to the left of   13
8     3  to the right of    5
9     3   to the left of    2
10    3   to the left of    8
11    3   to the left of    6
12    3  to the right of   11
13    4   to the left of    6
14    4           behind    1
15    4           behind   14
16    5   to the left of    3
17    5   to the left of    6
18    5   to the left of   13
19    5            under    1
20    6  to the right of    4
21    6  to the right of    3
22    6  to the right of   11
23    6   to the left of    9
24    6   to the left of    8
25    6  to the right of    5
26    6   to the left of    2
27    7  to the right of   10
28    8  to the right of    6
29    8  to the right of    3
30    9  to the right of    6
31   10   to the left of    7
32   11   to the left of    6
33   11   to the left of    3
34   13  to the right of    3
35   13  to the right of    5
36   14               by    1
37   14           behind    1
38   14      in front of    4