"The ticket is printing" vs "...is being printed"
The Middle Construction
"The ticket is printing" is something known as a middle construction. It is called "middle" because it is not exactly a passive sentence ("The ticket is being printed") and not exactly an active sentence (because the "ticket" is not really the agent of the action).
Some examples of middles from "An Introduction to English Syntax":
- This sweater washes well.
- These cars sold very quickly last week.
This sort of construction is extremely common. Sometimes, it conveys basically the same information as the passive, like in your examples about tickets printing. In that situation, either construction is completely valid in any context. "The ticket is printing" is just as correct as "the Boeing 737 flew to Seattle".
Another interesting thing about Middles
Sometimes, the middle construction conveys some additional information that neither the active nor passive conveys completely.
For example:
- I can't shut the window. (active)
- The window can't be shut. (passive)
- The window won't shut. (middle)
All of these sentences give the same fundamental information, i.e. that the action of shutting cannot occur with the window. But each one says something slightly different about why and to what extent.
In (1), the only claim is that the speaker cannot shut the window. In (2), using the passive, the implication is that nobody there is able to shut the window. In both cases, there is the possibility that perhaps the people trying to close the window are too weak or are doing it wrong. However, in (3), the blame for the inability to shut the window is placed squarely on the window: it is the window that won't shut.
By the way, the middle construction is not something that is unique to English.