IMO, a minimum of 2 1/2 feet

Though I have worked at grain elevators, and in that time period, I was never around even a steel or wood sheathed elevator in operation. I was at terminal grain facilities. That said as a caveat, the Farmers Cooperative Rural Grain Elevator as built has the filling shoot designed for loading boxcars. A person had to be able to manually, from outside the car, direct the nozzle of it towards the inside ends of the boxcar as it filled, and finally topping it off in the center where the doors are. It is not the loading equipment for a covered hopper. All that said, the distance issue can be considered based on that grain elevator employee handling the nozzle assembly during loading. He sure wasn't doing it standing in the doorway and he certainly needed some way of knowing where the grain was going inside the boxcar. Since the grain door, whether a wood "plug" or metal strapped cardboard, went up most of the way of the door opening, this limits the space for him to see inside the boxcar and for the nozzle to fit.

You might take a look at 

You want 4-6 feet spacing for the elevator

There needs to be enough space for the worker to comfortably walk on the ground between the elevator and car.  They need to be able to have room to hook up the car puller cables when moving cars, and other maintenance.  There also needs to be enough room to position a ladder on the fold-down platform to get up on the roof of a hopper, or to climb out of a boxcar once it's been coopered (grain doors nailed in).

The annex building can be located closer, since it usually stores bagged products that would be unloaded from boxcars.  That spacing would be more similar to a warehouse door, around 2-3 feet.

One beef I have with that particular elevator kit is that the doors on the truck unloading bay, are too low (at least for a typical Canadian elevator).  Typically the trackside door should be at boxcar floor height. The interior passage area is at the same level as the truck bay, which is either at the same level as the trackside door or a couple of steps down.  When I built my elevator, I used clapboard sheet to raise the truck bay to the correct height.  Here's a picture of my still-unfinished elevator.

Craig

UP standards

Here are the UP standards, third sheet most relevant.

0038.dgn (up.com)

Thanks for the input Gents.

Appreciate it.

Dave, I took that doc and calculated inches for 9', and then searched for Wisconsin Standards, instead of clearance or distance and got a similar doc for WI. Their clearance is a little tighter, at 8' 6" instead of the UP's 9'.

8' 6" is 102". Divided by 87 is 1.172".

So I'm going with 1 and 2/16 or 1 and 3/16 from the center line and we'll see how that looks.

Thanks again!

Edited for spelling