It would be very useful to have a setting in the ltn combinator to set the "encoded positions of ___ wagon" setting. maybe with a nice GUI where the user can select the positions and what should be there and it'll convert it into the number needed

Hi,

Train compositions are read-only signals, which are set once a train arrives at a LTN-Station. You can use them to de-/activate inserters at specific positions.

You cannot use those signals to define a train composition for a train stop:

Rejected features:
- limit trains by composition signal #93
Can't be done. Composition signal is ambiguous outside a specific stop.

https://forums.factorio.com/viewtopic.php?p=298016#p298016

cheers