Avoid it

And you've found the reason real railroads try to avoid having more than one industry on the same spur.  And the real world has room for more turnouts that we do on our layouts, which allows for more spurs.

Scott Chatfield 

Formerly of Southern Railway 

I suspected that was the case

I suspected that was the case but to paraphrase Donald Rumsfeld: you build a layout in the basement you have, not the one you wish you had. Turnouts and spurs suck up space like Covid in the ICU .

How was/is the movement of the cars on the siding coordinated?

You have to take into account that the local crews and industries were very familiar with their routines and usually had a good idea of when a car was ready to be moved and when another car would arrive....DaveB

As always...

It depends.

Most cases there was one industry on a siding, as Scott said, but having more than one wasn't uncommon and in one industry there might be multiple groups using the same track (inbound and outbound, boxcar and hopper car, all on the same track).  Also the older the era, the more likely there would be multiple industries on the same track.

Having said that there were multiple ways it could be handled.

• The conductor could call industry 1 before he left the yard and let them know they would have a car for industry 2, so be ready to clear up.
• The agent could call industry 1 and let them know they would have a car for industry 2, so be ready to clear up.
• The local could whistle as it was coming into town to let the industry know it was there so they would start buttoning things up.
• The local could operate on 2nd or 3rd shift when the industries weren't working.
• Industry 1 might be able to make its cars mobile in a couple minutes so they would just wait until the local got there.

If you think about it, industry 2 (or any industry on any track) is in the same boat, they can't be switched until their cars are buttoned up and safe to move, so its basically whatever process the railroad and the industries have agreed to.

Most places worked until the local got there and then got things ready, or they were already done with the cars by the time the local showed up.  Generally the locals switched the industries within a couple hours.  It was more consistent the closer the location was to the starting point of the local and gradually became more variable the further form the origin the industry was in time and distance.