I've been told to just make sure you have pictures of layout and rolling stock since it will presumably be burned or blown away by a tornado or hurricane.
If you line up the rolling stock and take a pic, then its not really a rush to get the estimated valuations.
Do you have a full replacement type plan? If not, I have always valued stuff at 50% of original retail, if I have the boxes, to account for depreciation. Locos may be worth that (not as much any more, with new ones being so much better) and its hard to sell old rolling stock, period, and never more than $10. Your initial estimates aren't bad, but IMHO, higher than true value on the open market.
As to the layout itself, you can either measure all the track and count and get original (or current) price of structures. I don't think the scenery counts as much, and it is cheap. Or (and these are old numbers) professional layout builders charge $500-600 per SF for total work, about half for plywood and track and wiring stage of completion. Taking a percentage of that, again probably 50%, might be acceptable as to the cost of doing it yourself.
Or do it multiple ways and average.
Lastly, you may want to consider if a disaster or a divorce is more likely......for the latter, you want it all valued as low as possible, presuming you take it as part of your half of the belongings.
