Please allow an option for a network id of zero.
The reason is to allow changing network id on the fly based on buffer threshold.
This way you can use positive and negative signals without trying to account for the default id.

Signal wires in Factorio do not carry a zero signal. Any signal with a value of zero is not emitted from devices. The behavior of Logistic Train Network when no signal is present is to use the default network configured in LTN. That is an Logistic Train Network specific behavior and not related to LTN Combinator Modernized.