My guess is never. The universal train icon for signs has a funnel and a cow-catcher, and it's a long time since a train looked anything like that.
Same goes for the universal sign for telephone - nothing like a modern cordless handset.
We can already seen this in action as the icon for 'mp3' on signs looks like a first-gen ipod. Already no players (even ipods) look like that, but I think it will stick around for a long time.
Some things get stuck as icons and the original meaning is lost with the sands of time. Once the icon is assigned to a meaning for most people, it sticks. There's no reason to update the icon, even if it no longer represents the object, because everyone agrees on the meaning.
Same goes for the universal sign for telephone - nothing like a modern cordless handset.
We can already seen this in action as the icon for 'mp3' on signs looks like a first-gen ipod. Already no players (even ipods) look like that, but I think it will stick around for a long time.
Some things get stuck as icons and the original meaning is lost with the sands of time. Once the icon is assigned to a meaning for most people, it sticks. There's no reason to update the icon, even if it no longer represents the object, because everyone agrees on the meaning.