Okay after considerable thought, the essential locomotives would be one locomotive from each of the 9 railroads I model with exception of the Pennsy & N&W with two each for those roads. Also essential, a Shay lettered for West VA Paper and Pulp.
N&W Y6b mallet Railking by MTH. This is the only articulated steam locomotive I own and is able to run on tight curves of my layout.
Canton Railroad SW 1500. The Canton is one of the short lines I model so this locomotive is certainly essential. Basically a switching railroad, the Canton, is the oldest railroad serving the City of Baltimore.
Pennsy scale GG1 both powered and dummy by Williams. An iconic locomotive type synonymous with the Pennsylvania Railroad. I saw these locomotives run, as a kid, on the Pennsy pulling both passenger and freight trains. As an adult I saw them on the point of Conrail and Amtrak trains. Two of these locomotives pulled a 23 car excursion train of Maryland school students from Baltimore to NYC and back in the spring of 1966. I was one of those students and I enjoyed every mile of the round trip too!
MA & PA SW 1 by MTH. My very first cab ride was in a SW 1, although it was a B&O SW 1. As the MA & PA is one of the short lines I model this locomotive is essential ... plus the cab ride memory is priceless!
Western Maryland BL2 by MTH. Although the WM has the ugliest livery ( IMHO as compared to other roads who purchased the BL2 from EMD ) for the BL2, I like the sleek exterior design of the this locomotive. The prototype WM owned 2 and both worked in Hagerstown yard until sometime in the 1980s. 81 is in the B&O Museum in Baltimore and I've seen it many times. WM 82 runs on a scenic railroad in West VA.
Aerotain by MTH. I've loved the Aerotrain since I first laid eyes on an edition of the Wonderbook of Trains. The book was in my elementary school library and the Aerotrain was on the cover. I thought the LT1200 locomotive was the coolest locomotive ever!! When MTH brought it out in O gauge I had to have it!
Patapsco and Back Rivers Baldwin VO 1000 by MTH. I love the look of this locomotive and am grateful MTH produced this model. The PB&R was owned by Bethlehem Steel Corp. and was the railroad that served the gigantic Bethlehem Steel Mill complex just outside Baltimore City. This was a rare find on E bay.
Baltimore and Annapolis SW 9 cow, calf, and caboose which is all that MTH produced in B&A livery. The cow has been upgraded to PS3. Although the prototype B&A did not own a calf, they did own a SW9 and caboose. Every Hour on the Hour, a book by John Merriken tells the compete history of the Washington Baltimore and Annapolis Electric Railway Company which eventually came to be the Baltimore and Annapolis Railroad Company. This MTH trio is a rare set, however rarity is not what makes it essential to me. Essential ... to my layout theme of Maryland's Class One and Short Lines of the post WW Two era.
Washington Terminal RS 1 with TMCC by Atlas O. I used to see the prototype of this locomotive working the Washington DC Union Station coach yard and it being serviced at the Ivy City Engine Terminal in the Amtrak era, circa mid 1970s. It still wore the Washington Terminal paint at that time. Later Amtrak painted in their livery. This model equipped with TMCC runs and sounds wonderful! The horn is close to prototypical. I prefer the ATLAS to the recent MTH WT RS 1 that was recently released by MTH. I also bought the MTH model but the ATLAS is essential to me. This ATLAS model was repainted in WT livery by a fellow forumite who posted it for sale here on the OGR Forum.
B&O passenger equipped GP-9 by MTH. GP-9 can do any kind of work that comes it's way. As an 11 year old kid I got a cab ride in a B&O GP7 which is just 250 hp shy of a GP9. The memory is vivid and I think of that cab ride every time I run this locomotive on my railroad.
Scale N&W J class 611 with TMCC by Lionel. I've loved this locomotive ever since the semi scale one came out in the Lionel catalogue in the late 1950s. To finally own a scale sized one is something I never thought possible. This model is the "Queen of my Fleet"!
Last but not least is the K line Shay. Aside from the fact I bought this locomotive from OGR Editor in Chief Alan Miller, this locomotive is just fun to watch in action. Equipped with TMCC the sound is great! I only run this locomotive when I have quests over to see the layout as parts are rare to non existent... and of course a Shay has LOTS of moving parts. A lovely locomotive with immense entertainment value and definite conversation piece.