I have insured my collection. My company is State Farm.
My basic home owners policy will NOT cover my collection. I discussed my hobby AT LENGTH with my agent. I brought him into the layout room. So often when you tell non model railroaders about what you do they have no idea what the reality is.
Once he visited the layout room and work shop, he then devised a standard template, for lack of a better term, for me to describe each piece in the collection.
I gave him a copy of the Walther's catalog and he accepted its published values for things like add-on detail parts (couplers, wheels, roofwalks etc.)
(This car cost this much, it has replacement wheels that cost that much, it has couplers that cost "x", the detail parts described as follows cost such and such. The decals cost "y"... etc, etc...)
Then we took pictures of the collection. Group photos were good enough, but I experimented with different number of cars in each photo. The initial photo of 450 black coal hoppers didn't seem to work with this guy. About twenty cars at a time seem to be best for me and my agent.
It took time. It was a big effort on my part (AND my Wife).
Once you get the bulk done then you simply have to add to it as the collection grows (They never seem to shrink).
Like most of you I have been doing this a LONG time, and you will probably surprise yourself with the dollar value you come up with. It WILL be much higher than you initially estimate.