Ever since I received my first transformer, a KW, in 1951, I have thought it common practice to never ever
leave a postwar transformer plugged in when not in use.
This caution was included in the Lionel postwar literature and every other reference I have ever seen.
When a ZW was left plugged in overnight at the museum where we were running the club layout, we returned next day to find the distinct aroma of warm insulation in the room, even though all handles were off. It's one of those "theoreticals", I guess. In theory it might be OK, but don't ever do it!
There are always stories about someone leaving a train running and then going away on a weekend trip, finally coming home days later to find the train still running just fine. But, then again, there are some people who win the lottery too!
Most modern transformers have an "on-off" switch which kills power entering
I've always thought it a good practice to have a master power switch which kills power to the entire train room, lights and controls. Makes it harder to forget that way.
Edited to change never
to never ever