I guess that means I'm stuck, unless Steam provides a way to install a prior release - I haven't knowingly backed up anything Trainz related, other than routinely saving new route and session versions as I edit them. Having been out of Trainz for a long time, and assuming the new issues were caused by the latest release, is it typical for a Trainz update to break significant program features, like being able to move existing rolling stock, etc.?
Not usually.
The inability to move rolling stock. You mean, the track doesn't act like track, or place track objects such as signals, switch levers, etc.?
This is a database-related bug and there's a workaround I came up with back in T:ANE that has finally affected everyone except for me. Seriously!
Go into Content Manager.
Click on the small arrow next to the word Filter: Installed.
This will expand the filter.
Click on the + sign located on the right.
You will see Asset KUID.
Scroll that until you see Category.
On the right side of that, change the category to track.
Highlight all your track by clicking on one and pressing CTRL+A to select them all.
Right-click on one of them and choose Open for editing.
Wait until all are opened for editing and then:
Right click and choose submit.
Any built-in or packaged assets can't be submitted. You will need to right-click on those again and choose Revert to original.
This should reset your track.