Hello,
Hope I'm not offending anybody, but the above is not correct - which is some of the sad parts with Trainz after TRS2004 for older users.
The default setting in TRS2006 & 2009 is
Alt + U to
fly and
Alt + Y to
walk.
For those of us with the old Alt + fly in our head to get to the fly mode, we end up with the walk instead.
I do it all the time, even if I'm getting slowly used to hold Alt button and just click on the U button to get to the fly modus.
Best wishes
Linda