Why? Because they are supporting their enemy, Ukraine. Doesn’t take a military genius to see the obvious motive.
It'd make more sense for it to be an operation Northwoods if anything, but that would be even more far-fetched.
In some intention, public opinion.
Sorry, but the high speed rail is Frances pride, and this was not some random attack. Russia absolutely could pull this off, and they have the motive. What other organisations does?