Hmm. Interesting question.

A diagonal crossing is defined by this Kata as below:

  \ /
   X
  / \

So I think the track you have drawn is either:

• not actually a "valid" track, or
• your "crossing" might be considered more like a bridge/underpass

Either way I don't think a crash can occur there.

It's already in the examples in the description, read them again.

...
helper.page_index(-10) # should == -1 because negative indexes are invalid