Hi, since sometime today I've had the same problem. I can delete tracks and streets just fine, but no assets; signals, buildings, trees, etc.
I have a wide array of mods enabled so I was unsure if any mod had caused it, but I've not done any changes to mods for a few days and have had no problems with it before. I own it on steam, running on build 13505, Windows 10 64-bit.
I've restarted a few times with no effect. Hopefully it magically fixes itself at some point.