As an initial step, I would create an inventory list of items in your collection. You can use a excel spread sheet to list the item m condition, presence of original boxes and other relative information. For Lionel items, you can list the two prices listed in the Greenberg guide , assign a value based on observed condition of the item, check your assigned value against recent ebay sales Sets and boxed sets should be considered as am individual line item . You might want to take a photo or two of each line item. So when you have all your items listed, you can sum up your high and low Greenberg values as well as the values you have assigned. The overall value of you collection will be in between the high and low Greenberg values in most cases.
Once you have completed this exercise you will have an idea of how large your collection is , what it looks like and a ballpark value of what might be realized at an auction sale.
I am familiar with an auction house on Long Island. The folks there handled the trains carefully , assembled attractive and balanced auction lots and seemed quite knowledgeable about items in the lots. . There is a sellers premium of 15 %. So as a seller you would realize about 85% of the winning bid. The buyer pays a premium , sales taxes and shipping . Depending on the size of the collection, the auction house may schedule several sessions to sell all the items . Takes a bit longer but the advantage is that the number of items sold in a session balances better against monetary resources of potential buyers.
I think that when you select an auction house, your choice might be driven by how comfortable and confident you might feel with the folks at the auction house. A feeling of trust and good will be an important factor in your choice.