LuaTransportLine vs internal transport line

[Gweneph]

I've been playing around with LuaTransportLines and it seems like every function only is dealing with the segment of internal transport line that is passing through the owner entity. Except for output_lines and input_lines where these seem to reference the LuaTranportLine whose owner is the entity where the connection is made to the next/previous internal transport line. These can shift around as the internal transport lines are merged or split.

In any case, I was wondering/hoping there might be a way to do something like get_internal_contents() that would return the contents of all the items on the internal transport line.
Or, if there was a way to easily get the LuaTransportLine that the items I can count would go to next, like output_lines and input_lines would give if the internal lines never merged.
Either of these would make it a lot easier to search along a line to find mixed belts, or count summaries per lane.