Well after reading the discussion about having an inventory, I decided I should probably figure out what I have. Not done yet, but I think I bought way too much stuff for my grandkids. 
I have a pretty good handle on the engines but rolling stock is another matter. Still have a couple of more shelves to do but in 3 years I think I will have accumulated 30 engines and 140 cars. When I did my count, if I had a 6 car coal hopper set, I counted that as one. Same with passenger car sets, that is a one. Diesel ABA is also one. Funny I started out buying steamers and now have only 3. I still have a couple more diesels on order (BN) and one of the Big Boys. I am not going to count my post war or 027 cars and engines. Those are for "under the tree".
I guess my question is when do you know you have enough stuff? OK my absolute limit would be needing to rent a storage unit to keep trains - for me that IS TOO MUCH. So far I have been good at selling off items that did not fit what I wanted to do. Two I am stuck on - A Pennsy GG1 and passenger set and a Norfolk and Western Passenger set waiting for a scale J class 611. As much as I love the history of the Eastern railroads, I am finding myself building roads Chicago and west, so the Eastern roads do not seem to fit. Nevertheless, I am likely to hold on to the Pennsy and N&W roads as an homage to the GG1 and "J".
And on another note, the CEO asked me if I could turn one of my buildings into a US Marine Corp recruiting office. My oldest spent 9 years in (enlisted and out as a Staff Sgt) and I agree it would be a nice touch. Anyone done that? I know we have several Marines on the forum.
Jeff
