I often hear both variants:
Der Zug ist scheinbar schon abgefahren.
Der Zug ist anscheinend schon abgefahren.
Is there a difference in meaning or can both expressions be used interchangeably?
Mixing them up is a common mistake. In fact so common that the Duden decided to add the definition of "anscheinend" to "scheinbar", too. It's marked as "selten", though.
"Anscheinend" is used if something looks like it is the case. "Es hat den Anschein."
You think he is sleeping.
"Scheinbar" is used if you know something is pretended/simulated. "Etwas zum Schein tun."
You know he is not sleeping, just pretending to do so.
In your first sentence the train is faking its departure. In the second it looks like the train has already left.
I think when you ask most native speakers if they are aware of the difference, the answer would be no.
They have to look up the difference and it doesn't really matter to them.
Still, if you value the subtle differences, it's good to know them. "Anscheinend" does not make a statement as to the veracity of the appearance, while "scheinbar" does. A synonym for "anscheinend" is "augenscheinlich" (with maybe a hint of a doubt in it).
There are also some more subtly different variations, "offenbar" and "offensichtlich": The appearance is obviously true.
Everyone can see that he is really sleeping.