TL;DR
The train stop GUI should have an additional field to limit what configuration of trains can arrive at the station.What ?
In the GUI for train stops there should be an extra window to configure what kind of trains are allowed to choose this station as their destination. Like so:Each row represents a train configuration. First row is a train 2 "Locomotive 1", 3 "Cargo wagon 1" and 2 "Locomotive 1". Those come from Nullius where there are multiple levels of locomotives and wagons.
The second row uses a short form for longer trains using a count for repeats. The user can use a count or not. The locomotive with a red background (comes from LTN) is a virtual signal denoting any locomotive. One usually doesn't care what type of locomotive a train has but for cargo and fluid wagons the size can be critical. The orange background denotes that the item is currently selected and the count could be adjusted in the count input field.
The input matrix should expand to the right and down to always allow an extra wagon or train to be added. Although with the count feature there hopefully won't be really long trains to enter. The matrix should allow adding any type of locomotive, cargo wagon or fluid wagon. Also the virtual signals "any locomotive", "any cargo wagon", "any fluid wagon", "any not locomotive" signals. For each locomotive there could also be a virtual signal for the locomotive in backwards direction, which would have the icon of the locomotive mirrored. Alternatively a negative count could be used to represent backwards locomotives.
As an optional special case a count of 0 could mean any amount. For example a refueling station could accept trains with "Locomotive" + 0 "not locomotive". Meaning any train that has one and only one locomotive at the front.
Why ?
As one starts one builds simple short trains. One locomotive, one cargo wagon. As one expands there is demand for more goods requiring larger trains to cope with the amounts. But a station designed to handle ore trains with 1 cargo wagon won't fare well if a train with 3 cargo wagon arrives. So currently one has to use all new names for mines, ore smelter, oil pump jacks or oil refineries to include the size of the trains they service. By adding this feature one could keep a consistent naming scheme across all stations but still have the right trains arrive at the right stations.Other stations that would greatly benefit would be refueling stations. For them it is critical that they know where the locomotives will be. They will be users of the "any not locomotive" signal for their train constraints.
Last, with the backwards direction locomotives, it would allow protecting terminal stations where trains without a backwards facing locomotive would become stuck.
And then when there are mods that introduce different kinds of locomotives, cargo wagons and fluid wagons it becomes a whole new game for station designs. The inventory size of wagons can change and bad things happen when the buffer chests or tanks aren't matched to the train. Or the size of locomotives or wagons can change. Now the inserters and fluid pumps need to be a totally different locations.
This feature would allow making the train network more robust and more fun. Allowing for a mixture of train configurations without having to mangle the train stop names to denote train types as well as the stops function.