I was playing online with a friend without problems until we built our first train intersection, where a desync occurred. A few junctions later, we figured out the reason. Should be pretty easy to reproduce, however, in some rare cases and when rotating too much no desync occurred:
>be client on a server
>find a spot where a rail signal can attach to more then one rail:
![Image](https://forums.factorio.com/images/ext/bfefc05070928399a119027f98312241.jpg)
>select signal, hover over said spot, now rotate until it connects to the non-default rail
>place signal:
![Image](https://forums.factorio.com/images/ext/8a8f03f3e6db1a4834ade751fd6efb7c.jpg)
[at this point client and host should see the signal connected to two different rails]
>deconstruct signal BAM desync:
![Image](https://forums.factorio.com/images/ext/d14c049387a0bca9de774bc66475b633.jpg)
>after redownloading the map, the signal is now attached to the same rail as the host saw it:
![Image](https://forums.factorio.com/images/ext/9e82fabe4b3afdbd5c55551fc61bb44b.jpg)
Greetings,
- Sauerkraut
EDIT: Desync report.