As for it being a precision metering device... The metering could be rough, like with stack granularity. So if you signal 536 you get 600 items. Or it could be randomized as 536 + rand(50). You can close the door before the loader storage is empty but it's not a precise count. At the moment the doors close at exactly 4000 iron plates. That seems inconsistent with your explanation.
I've done some more thinking about it, and one possibility could be to expose a lamp-like connection with simple enable-disable configuration. Inserter stack size bonus would make the metering imprecise. Having looked at videos of actual hopper-based rail loaders in operation, the flow can be readily shut off once the cargo wagon is full, which diminishes the rate of material flow as there is no longer empty space to be filled. So it's easy to stop the loader once the wagon is full, but difficult to do so with a partial load (unless the loader itself empties), and even harder to do so precisely.
Have you checked lo-res and hi-res sprites? I'm using Bobs+Angels+AAI so I have wooden and cement rails. They look a lot different and I don't think they replaced the vanilla rail images, just added the wooden ones.
I haven't checked how you made the unloader graphics but it sounds like you just made one image. Take a look in data/base/graphics/entity/straight-rail/. The games rails are made out of several layers of images. If you do the same and use the original images for the actual rails then they will match perfectly and you don't even have to draw them. Hope that helps.
The original rail graphics are (naturally enough) 2x2, so I stitched them together to make 6x2 images for use with the unloader. I'll have a look again and see if I can figure out what's going on with the "metals" layer, which appears to be the one with the most apparently visual difference when layered on top of the grate image. I might be able to do something more complicated at runtime with blending mod graphics provided by BioIndustries and other mods that potentially add other straight-rail variants, when I get some time.
The thing is that at the start/middle of the game the loaders/unloaders are faster than using inserters. But towards the end of the game they will be slower. That is a bit unrealistic. Letting things just fall in/out of a train car should be much faster than picking up each item (even considering stack bonus). On the other hand having a loader that is 10000 times faster than a inserter when you haven't researched even stack inserters is unrealistic game wise. That's why I suggested tho have loaders with different speed.
I'll look into doubling the overall speed. Since speed scales with stack inserter research I'm not too concerned about the loaders being too fast in the early game. But I'm also not particularly concerned that they are presently slower than 12 express stack inserters. That's a big investment both in materials and space, and I feel the player should see some benefit from that.
What you say about the 100+ ton wagon makes sense. But I bet wooden rails aren't what you think. The rails are not made out of wood. Instead the rails are fastened on wooden beam. The more advanced rails use concrete beams instead of wood. In the unloader those beams are replaced by the grate. Both rail types use steel for the rail itself. So wooden rails or cement rails makes no difference to the design of the unloader. The part that differs is what you throw away during construction.
Thing is, when you research railraod all you get is wooden rails. The cement using rails need a lot more research and infrastructure to build. Not having a recipe to build loader/unloader using wooden rails makes them a lot more complicated to manufacture.
Note: Wooden rails are from "Bio Industries > Angels refining".
You are correct. I've played with Bob's+Angels quite a bit, but never used Bio Industries, so I'm unfamiliar with where wooden rails fit in the tech progression. I was thinking they were a lower-tech alternative, perhaps with limited top speed, for use in the very early game.
I have compact setups with warehouses that handle multiple goods. For example I have a crusher factory with 2 train stops. At one side all 6 ores come in by train and get dumped into a warehouse. From there filter inserter put them on belts to the proper ore crusher. From the ore crushers the results go into a single warehouse again for distribution. LTN then sends a train to pick up 3000 saphirite + 2000 stiratite in a train and those gets loaded.
With the loader I would have to pre-sort all the output from the ore crusher and make one station per ore + one for crushed stone. That's a 7 fold increase and simply doesn't fit in my base.
This sort of setup should work fine with Bulk Rail Loaders as long as each train contains at most 5 item types. If you have separate trains like your saphirite + stiratite, the unloader will handle those two item types, and then handle two (or up to 5) different item types when the next train arrives.
When I make a blueprint of a station with loaders or unloaders, place that blueprint and walk along it with the blueprint in my hand then bluebuild rewritten will place entities within my reach. When it places a loader or unloader the blueprint in my hand disappears and I suddenly hold a loader or unloader instead. It also fails to place the loader/unloader maybe half the time. Can't test yet if it happens with construction bots too.
Thank you for the report on Bluebuild. This is a case I can handle better so you don't lose your blueprint.
A loader/unloader also can't be replaced by the other like you would replace a wooden chest with a steel chest. The advantage of replacing one with the other would be that the contents would transfer.
I usually don't have space in my inventory for 50k iron ore or such.
Bulk Rail Loaders are quite tricky because they are placed directly over existing rails, and Factorio isn't really designed for that. Fast-replace in particular is not something I know how I can implement given how the mod API works.
EDIT: The problem with blueprints being destroyed is caused by the failure to place. And the failure to place seems to be that when blueprinting a loader/unloader any tree that is in the way does not get marked for deconstruction. Placement of the loader/unloader then fails because there is something in the way and the loader/unloader is returned to the players hand. This replaces the blueprint with a loader/unloader instead.
I'll see what I can do around automatically marking trees/rocks in the construction area for deconstruction. This is something that Factorio handles automatically for "normal" entities and blueprints, but Bulk Rail Loaders are far from normal.
Note: The unloader requires 3 rails to build but it actually only covers 2 rails. Is that intentional?
Rails are 2x2, but cargo wagons are positioned every 7 tiles (6 tiles for the wagon itself, plus 1 tile gap between them, which is where you normally place power poles for your stack inserters). This means that depending on the station and the particular wagon position, the loader overlaps either 2 or 3 rail segments.
2) I think the collision mask is wrong leaving trees behind that should be deconstructed when placing a blueprint with shift.
If the collision mask correctly reflected the final footprint of the (un)loader, it wouldn't be possible to place them over existing rails. I've also experimented with having the (un)loaders place their own rails, but past attempts made for a very confusing experience because of the way they don't align with the normal 2x2 rail grid. I may look at that alternative again, as the tradeoff is more predictable behavior when using blueprints.