When can LuaTrain.front_rail be nil?

[NiftyManiac]

In what cases will calling train.front_rail or train.back_rail return nil? Since it's impossible to place a train without rail I'm curious so things don't break unexpectedly.

As a side note, is there any direct way of getting the unminable rails underneath a wagon, short of getting all nearby rails with find_entities_filtered and attempting to destroy() them?