I have seen one layout with a coal tipple, and I'm trying to explain how I think it works to you.
So basicly you have a large silo above the tracks; which is probably the most common type of modern coal loader. The silo has a track through it, and there's a conveyor running from the top of it into the hillside behind it. There's an optical sensor between the rails underneath the silo. When you run a train into it, there's an electrical switch that's off for the locomotives.. When the first coalcar is positioned under the silo, you flick the switch to "on". When the optical sensor senses something above it (in this case the coal car) then it activates the conveyor into the hillside. The conveyor carries coal into the hollow silo, where it falls the lenghth of the silo; through a funnel, and into the hopper car.
When the optical sensor sees nothing, (i.e. the gap between cars) then the conveyor is not on.
That's how I'm guessing that this works. Someone else probably knows better. Good luck!
If you are a paying member of Trainorders, there was a nice detailed series of post by someone over there that built an operating version of the Premier Elkhorn floodloader at Myra, Ky (CSX). You would need to search the archives to access these post. From what I remember, the outlet (chute) was controlled by a gearhead motor driving a shaft rotating inside a tube. The tube was milled open over the chute and the shaft was mill with a flat spot to line up with the opening in the tube. The structure above was a bin holding plastic pellets. As the gearhead motor rotated the shaft, a measured amount of coal would be dispensed per each rotation or it could be jogged 90 degrees off the close position and the pleets could drop freely through the opening. His comments were this usually caused jams and dispensing worked better by letting the motor rotate to dispense a set amount of pellets with each revolution. Personally, if I were to do this, I would use a chute with a sliding trap door controlled by a stall switch machine. Real tipples use this setup except the sliding outlet door is driven by compressed air. To load, slide the door open, the slide closed to stop the flow. Use the remote mounting hardware with the cable to remote mount the switch machine. The cable could be painted to look like conduit for power running into the tipple.
I've never seen anyone do an operating conveyor that works. Most belts are 48 inch max and it would be too hard to get a flex material that width in HO scale to form a "V" to conform to the required angled rollers. Anything that small with enough friction to be roller driven would need to be stretched tight and the surface would be flat allowing the coal material to fall off the sides when moving. There would be ways to overcome this but your conveyor would look nothing like a scale conveyor. The option here is to use a 1" diameter PVC pipe to represent the enclosed dust shields used around conveyors. Inside you would place an auger spun by a geared motor such that the pellets of coal would be augered up through he PVC pipe and fed into the loader holding bin. I've seen two done like this and they used big auger drill bits bought from Lowes and allowed to float inside the pipe.
In planning your layout, remember that NS only has a few hundred feet of mainline trackage in Eastern Kentucky. Everything else is just branchlines that cross over the Tug from the Pokey main in WV. There are no NS passing track in EKY, the branches are all single track except for under the many loaders. The Wolf Creek Branch in Martin Co. would be the best one to model as it does have small yards Martikki and Pontikki where you could tie a train up for a meet.
Robby Modeling the L&N CV Subdivision in 1978 http://s226.photobucket.com/albums/dd247/robby-ky/CV%20Subdivision%20Layout/
About 5-7 years ago in MR (I'll see if I can find which edition when I can get to my copies) there was a step by step project of a working coal tipple. They used 1" pvc with a tee and a 1" spiral drill bit inside. They hooked it up to an old car windshield wiper motor and a couple of parts from Rat Shack to control the flow and speed. I t was pretty cool. Always wanted to build one but never had the need.
Hope this helps
Modeling the Klamath River area in HO on a proto-lanced sub of the SP “The State of Jefferson Line”
This used to work, back when I bought it over 40 years ago:
It's an ancient Vollmer kit. The two chute doors are solenoid-activated. The coal is loaded into the building by removing the roof. It holds about 2 of these old hoppers worth. The solenoids still work, but I lost the core to one of them, and I have to fabricate another one from a lock blank. Also, the hinge mechanism is broken on one door. They are fragile enough that I'll probably rebuild them both to get a more robust model.
At the other end of the line, my hoppers can be unloaded here:
These are old Mantua "clamshell" hoppers, named after the non-prototypical doors on the bottom of the cars. They do actually work, though, when the car passes over a special track section which opens the doors mechanically. Here, the coal falls into the "heap" of coal below the trestle. If you look closely, you'll see that it's actually more like a volcanic crater. The coal from the hopper ends up falling through to a box below the layout, where I can remove it and re-load the cars later.
It takes an iron man to play with a toy iron horse.
I didn't get to my issues yet but a quick search on line shows it to be July 1999.
"Build a working coal tipple PVC pipe, an auger bit, and a windshield wiper motor are key ingredients Jim Ferenc"
Too bad we can't scale the laws of physics. The sight of seeing modeled coal fall 87.1 times faster than scale is unappealing.