The "transition era" remains a popular era to model because of the mix of steam and early diesels. I've seen it defined as into the '60s, but for many modelers, it's the '50s. If you're modeling the New Haven's West End, however, the transition era mostly concludes at the end of 1948 as steam was either scrapped or moved east, and system-wide would end in 1952.
On the West End (New Haven and Hartford Divisions) in spring of 1949 all of the remaining steam is listed in yard work in Maybrook or Cedar Hill: #3005 is on a Cedar Hill tool train, and #3020 is on a Cedar Hill emergency-work train, and #3207 works the Maybrook Hump, #3405 is in Maybrook work service, and #3402, 3410, 3411, 3413, 3419 and 3434 are working Maybrook yard until 1950. All the rest is moved to the Providence or Boston Divisions. There are a number of locomotives listed in Cedar Hill as D.S.F, which I understand to mean "dumped, save fuel" and are therefore not in service. I'm not aware of any modelers with the space to model the Cedar Hill or Maybrook yards though.
Diesel SwitchersFor diesels, switchers led the way in 1931 with ALCo Demonstrator #600. Although ALCo originally intended to demonstrate the new locomotive on many railroads, the New Haven extended their test, then purchased the demonstrator itself, numbering it New Haven #0900, later classified DEY-1.
The New Haven classification system for diesel locomotives was instituted in 1944, although in general practice it appears that many railroad employees still just referred to them by road number, such as "the four-hundreds" when referring to the DER-2a/b (ALCo FA-1, FB-1) locomotives. In addition, diesel locomotives delivered up to 1950 were numbered with a leading '0' to differentiate them from steam road numbers. They didn't change the numbering of these locomotives, but new ones were numbered without the leading '0' after this time.
In my case, the 44-tonner is the primary diesel switcher, with two assigned to New Britain. ALCo S-1 and S-2 switchers are featured on road freights, but in different years.
GE Custom SwitchersNH Class: DEY 2 (0901-0905) and DEY-2a (0906-0910)
The next diesels to arrive following the purchase of ALCo Demonstrator #600 in 1931 were ten switchers built by GE in 1936/37. These locomotives saw assignment to Boston, Providence, and New Haven, but all working Cedar Hill by 1948.
Joe Smith covered these in detail, including his amazing model, here. I don't believe any brass models were ever produced.
ALCo High HoodsNew Haven Class: DEY-1 (0900), DEY-1a (0911-0920), DEY-1b (0921-0930)
As noted, ALCo Demonstrator 600 was purchased by the New Haven and in service since 1931. Going back to ALCo in 1938, they acquired ten of the new production versions of #0900, the HH600 locomotives in 1938, and in 1939/40 ten more of the upgraded HH660. By 1948 these were all assigned to switching service in the Boston Division, although they occasionally served on road duty, as noted in the Engine Assignments for April 1949 where #0900 was assigned to the Taunton-Dighton local and in September 1949 #0923 was the Bird Mills Local.
Having said that, NHRHTA released a special run of the Atlas model in the Warm Orange and Hunter Green scheme that looks fantastic on this model. So it will make periodic visits to New Britain (especially since it's already DCC-sound equipped). For the foreseeable future it will be on lease to Stanley Works.
Brass models have also been produced in the past.
GE 44-tonnersNH Class: DEY-4 (0800-0818)
In 1941, the beginning of the end of steam on the New Haven really took hold, with the first ALCo S-1s (see below), ALCo DL-109 dual-service road locomotives, and the first 7 GE 44-tonners.
The two local New Britain switchers are DEY-4s. Assigned switchers from NH documents and Engine Assignment books (always looking for more!):
1948: 0802, 0812
1949: 0802, 0805
1950: 0805, 0806
1952: 0804, 0810
1956: 0802, 0809
1957: 0802, 0809
I covered the DEY-4 locomotives and available models in detail here, here, and here
ALCo S-1NH Class: DEY-3 (0931-0995)
The DEY-3 became the largest class of diesel locomotives on the New Haven, with 65 built between 1941 and 1949. While primarily assigned to switching duties across the system, they were also used on local freights, such as the Hartford-New Hartford local (HDX-5). This train was discontinued by mid-1948, when it was usually hauled by K-1-d #479. Reinstated in September 1949 with #0967 assigned, and listed in the October 1950 assignments with #0994. I'm currently missing the spring 1950 information, but it was discontinued again by spring 1951.
There's also a J.W. Swanberg photograph of #0967 in New Britain on December 1, 1961 with a caption that says it's replacing the usual 44-tonner on that day. However, I think it's on NX-28, the Cedar Hill-Collinsville local, since it was assigned that service in 1957, it's not a stretch to assume it was still on that local in 1961.
Available in brass and Life-Like Proto 2000/Walthers Proto.
ALCo S-2NH Class: DEY-5 (0600-0621)
In addition to replacing yard steam, though, they also started replacing road steam. To model a steam-only operating session, I need to go back before July 1945 when DEY-5 #0605 and #0606 replaced the J-1s on the NY/YN Cedar Hill/Holyoke freights. They continued in this assignment (occasionally including/replaced by #0604 as documented here) until the second delivery of the ALCo-GE RS-2s (New Haven class DERS-2b) in 1948.
Available in brass and from Atlas.
Lima-Hamilton SwitchersNew Haven Class: DEY-6 (630-639*)
Purchased in 1950, these 7 locomotives replaced the Y-3s and the L-1 hump locomotive. Over the years they did see some service elsewhere, but they are more closely associated with Maybrook yard.
I don't believe these have been produced in brass.
EMD SW-1200sNew Haven Class: DEY-7
The first EMD locomotives purchased by the New Haven in 1956, the 20 DEY-7 switchers were equipped with the optional Flexicoil trucks to allow road service use with 65 MPH gearing. The upgraded Walthers model is expected to ship in 2019.
Brass models have also been released.