limit trains by composition signal

[Sil3ntStorm]

There are two kind of trains available to LTN, a 1-1-1 and a 1-2-1.

The target station outputs 5 for encoded locomotive location, but LTN keeps creating the order on a 1-2-1 train, which doesn't match the locomotive locations asked for.

It also created an order on a 1-1-1 afterwards for a target station requesting a locomotive location of 9 and cargo wagon location of 6, which is asking for 1-2-1 trains.

LTN 1.6.1 on Factorio 0.16.10

[0ptera]

The problem with using composition as filter instead of train size was explained in an post in the old discussion thread:

Train composition can be used to service different train sizes and configurations in one station.
Why is it not a input?
Train composition is directional with the LSB facing the stop. When leaving a station trains change their front and back depending on track layout (RoRo vsTerminus) and the signal would become ambiguous.

[Sil3ntStorm]

so you're saying that having these composition signals is basically useless as it doesn't prevent trains that mismatch the station design from getting sent there.
I could have a train with say 1-4-1 and a train with 2-2-2, which both are the same length getting sent to any station. Which would mean that the 2-2-2 train would be sent to a station dealing with 1-4-1 trains, or even worse the other way around. As such the station won't be able to unload the train, nor would it necessarily be able to unload the requested amount from such a train as it won't fit into the limited cargo wagons matching up with the unloading equipment.
With the addition of artillery wagons, one may even add those to some trains which could also become an issue quickly.

I guess you're going to say one can do things with the outputs but rather than setting up overly complex, large (and expensive) circuitry at every station dealing with decoding the signal then disable some inserters / pumps it'd be much more fitting to do that in the request and only send matching trains.

[0ptera]

Composition signals are read only signals used to create stations capable of servicing multiple compositions
Have inserters capable of serving any composition and activate only the ones at positions with cargo wagons. It requires 4 additional combinators + 1 per cargo wagon position, I'd hardly call that overly complicated.

[labmonkey42]

I still wish for something like this in order to differentiate between solid and fluid trains. While I think I understand the forward/reverse ambiguity argument, how about the following logic:

1. read the composition of each available train
2. for each train, read the composition of that train in reverse order, discarding that train its composition is asymmetric
3. attempt to select a train by composition from any remaining trains
   This means that selecting trains by composition would not work for asymmetric trains, but that's no different from the current behavior.

Using similar logic, a train could select a depot to which is should return:

1. compare train composition with reverse composition
2. if composition is symmetric, select a depot by composition; else select by current logic

Apologies for rehashing an old issue here, but I'd really like to see this work in order to avoid composing trains in which length indicates composition (primarily in order to standardize station blueprints).

[0ptera]

Since 1.7.0 stations can be assigned multiple network IDs.
Practically there's no need for limiting by length or composition any more.

[labmonkey42]

Fair enough. That handles every concern I can think of which would make use of train composition as input. Thanks for the feedback.