Oh this works great! You are a star.
I have noticed one oddity though. If a train pulls in to a station with no barrels of the right colour aboard, the loading doesn't activate. I noticed it after I moved the unloaders to the same side as the loaders so maybe it's me.

You're a star.
You're also spot on about the either side idea. Failed miserably

Thanks for the advice but I need the barrels to differentiate wanted destinations. When a train pulls in to a red station ALL the red barrels get off. The waiting red barrels get on. Train leaves and gets to yellow station where ALL yellow barrels get off and waiting yellows get on. And so on until ...

Hi gang,
I'm building a passenger railway for no reason other than to see if I could (I can't :-)
I'm using barrels as people and the contents as destinations. So a blue barrel (water) will go to the blue station, get off, wander around on a conveyer for a bit, then come back to the blue station to ...