Introduced: 1990 (Kato/Japan SD7/9) 2004 (Atlas "Classic" SD7/9, Atlas "Master Line" SD24 and SD26) and 2023 (DCC-Sound SD7/9)
These models (SD7/9, SD24, SD26) have all shared the same mechanisms over the years, so to save myself a bit of time I'm going to cover them all here.
The 1990 Kato-made SD7/9 was the first Atlas diesel to feature a so-called "low friction drive" (an innovation first introduced in Kato-USA's U30C model of the previous year). Kato's idea was to reduce current-collection friction in the trucks by mounting pointy-ended axles between two dimpled phosphor bronze wipers. Said wipers in turn transfer current to the split-frame chassis by contacting long phosphor bronze strips sandwiched between the fuel tank and the frame (a convention that remains a staple of N scale locomotive design for decades).
In addition to the low friction drive, the rest of the mechanism employs most of the other time-honored design features one normally associates with "modern" Atlas diesels - split-frame metal chassis, dual-flywheels, plastic truck assemblies, plastic gearing, blackened wheels, all-wheel drive, and all-wheel pick-up (no traction tires). The motor is an open-sided 5-poler. Directional lighting is provided by PC boards mounted on either end of the chassis. The couplers are truck-mounted Rapidos (open pilots). Wheels are low-profile and have no problems on Code-55 track.
Performance on these models is perfect in every way - smooth, quiet, flawless pickup and throttle response, exceptional pulling power, etc. About my only complaint is with the open pilots and truck-mounted Rapido couplers. Although at this stage of the game, most of these have long since been converted to Micro-Trains couplers (said conversion kit coming with full pilots and shell-mounted couplers). Oh, and I suppose one could grouse about the excessively high top-end speed. But, come on, that's just nit-picking.
Atlas eventually severed their ties with Kato in the mid-90s, and in 2004 this model was re-released under the "Atlas Classic" line (redesigned/retooled Chinese-manufactured remakes of the locomotives originally manufactured by Kato). The new Atlas Classic shells are somewhat narrower than the earlier Kato-made shells (and thus will not fit on the earlier chassis). Said new shells include full pilots and shell-mounted Accumate couplers -
The chassis/mechanism is completely new as well -
In addition to shell-mounted couplers, these Atlas Classic locos are fully DCC-Ready (available with factory-installed decoders even). They also feature Atlas's "scale speed" motor. The lightboard comes with "golden white" LEDs. And as nice as the old the old Kato-made models ran, these new ones run even better (just a bit smoother and quieter).
2004 also saw the release of SD24's (both high and low hood). Both employ the same basic chassis/mechanism as the Atlas Classic SD7/9) -
2004 also saw the release of SD26's (and yes, same basic chassis/mechanism as the "Classic" SD7/9 and "Master Line" SD24) -
Atlas released all-new SD7/9 models in 2023 (now under their "Master Line"). These were available either DCC-Ready or with factory-installed ESU LokSound decoders -
This was the first diesel model that Atlas completely redesigned when adding sound. Rather than using the old split-frame chassis to transfer current to the main PC board, the chassis is now essentially one big hunk of metal with wires from the trucks transferring current directly to the board. Additional wires from the board transfer current to the motor contacts. LEDs mounted to either end of the board provide directional lighting. The drivetrain components drop inside of the chassis and a separate metal lid screwed to the top holds everything in place. The main PC board is screwed to the top of the chassis lid. The main PC board comes with a preinstalled speaker on all models. The DCC interface is designated as "E24" and is (unfortunately) proprietary to ESU. The upshot there is that swapping in some other brand of decoder is not going to be a simple operation.
Everything else (trucks, shell, etc) looks to be the same as on the previous version. The only noteworthy difference is that the arm on the piece of light-conducting plastic for the number boards and headlight is a bit longer on the newer version and does not work with the previous version (it gets hung up on the headlight LED). So, if you want to swap shells between the two versions, leave that particular part with the chassis from whence it came.
Performance on this new version is outstanding in every way and every bit the equal of the previous version. The sound feature is particularly impressive.
Prototype information (SD7/9) -
The Atlas Classic N Scale SD-7 Locomotive is based on the prototype manufactured by EMD from February 1952 to November 1953. Utilizing a 1,500 hp engine and a longer underframe than the GP-7, the SD-7 was the first locomotive to use EMD's Flexicoil truck. Built by EMD between 1954 and 1959, the SD-9 6-axle road switchers produced 1,750 HP and were used primarily for freight service throughout the US, both on main and secondary lines. Many SD-9 locomotives are still in service today, serving as road switchers for shortline & industrial railroads, and as yard switchers for major "Class 1" lines.
Prototype information (SD24) -
With over 220 units produced by EMD between July 1958 and March 1963, the SD24 locomotive generated a total of 2,400 hp with its turbocharged 567D-3 prime mover and C-C wheel arrangement. The majority of SD24s produced were purchased by four roads: Chicago Burlington & Quincy, Santa Fe, Southern, and Union Pacific. Atlas' NEW N scale model features details unique to each road name:
- Burlington (CB&Q): High nose, single-chime air horn on short hood, Gyra-light on short hood (non-operating),
- Santa Fe: Low nose, 3-chime horn mounted on left side of hood behind cab
- Southern: High nose, 5-chime horn mounted on cab, bell detail on short hood
- Union Pacific: Low nose, 3-chime horn mounted between 2nd and 3rd radiator fans, winterization hatch
Prototype information (SD26) -
In early 1973, Santa Fe instituted a rebuilding program for their large fleet of SD24s; the end result of this program was the SD26. Fairly extensive modifications were made to the locomotives, making them visually distinct from stock SD-4s. The addition of an inertial air filtering system created a 'hump' directly behind the cab. This, in turn, caused the 4 roof-top air tanks to be separated and relocated further back along the edges of the roof. A 'second generation' EMD-style traction motor blower bulge was included on the left side of most rebuilt locomotives. Other spotting features included the addition of a cab-top beacon and air conditioner on most units, as well as the relocation of the bell to the roof. Most SD26s were retired by Santa Fe in 1985 and 35 of these units were sold to Guilford Transportation Industries in 1986. Of this group, at least 3 were still in service for Guilford as of January 2004, over 44 years after they were built.
Shell Removal -
Removing the shell on all these models is beyond simple - grab the fuel tank with one hand and the shell with the other and it should slide up and off quite readily (just give it a bit of a wiggle).
Trivia - SD7/9's are generally referred to as SD7/9's because the external differences between prototypical SD7's and SD9's are negligible.
Model Features (2023) -
- Powered low friction drive
- Blackened metal wheels
- Dual brass flywheels
- Directional lighting
- Golden-white LEDs
- Body-mounted AccuMate couplers
- Atlas Silver Sound Ready models are equipped with a speaker for easy conversion to sound with the addition of a decoder
DCC Features (2023) -
- Supports all DCC-programming modes
- Includes RailCom and RailComPlus, with 14, 28 or 128 speed steps and with 2-digit and 4-digit addressing.
- Flexible mapping of function keys F0 to F28.
- A total of 6 DCC function outputs are available, and all can be function mapped (disable, brightness, light effects) individually
- Follows all NMRA DCC standards and recommended practices
- Over 20 sound effects are available, including engine start-up and shutdown, prime mover sounds through all eight notches, bell, air horn, air compressor, dynamic brakes and more
- Up to 16 user-selectable horns, 2 user-selectable bells, and 2 user-selectable synchronized brake squeals.
- Manual and Automatic Notching modes with the ability to change modes "on the fly" are provided for true realism.
Grade: A (for all of 'em)
Kato-made SD7/9 reviewed: 6/90 Model Railroader ("Once again, Atlas has teamed up with Kato of Japan to produce a pair of superb EMD SD7 and SD9 diesel road switchers. This joint project includes four distinctly different road switchers that come decorated for ten railroads, with two engine numbers offered in each scheme... With the exception of the tiny classification lights over the number boards, SD7s and SD9s are virtually identical in exterior appearance. The SD7s have these lights centered over the number board, while the SD9s have their classification lights near the outer corners of the hoods...
"The Atlas model closely follows the dimensions shown in the (drawings), measuring a scale 10'-6" wide, 15'0" high, and 58'-6" long (over the end sills). It has the correct 35-foot truck centers, and the hood has the proper scale 6-foot width. These locomotives follow Kato's proven design. Each one has a highly detailed molded styrene body that is made up of several individual castings: body shell, cab, cab-window inserts, handrail/walkway assembly, and sill/pilot casting. Two zinc-alloy frame halves enclose the motor, flywheels, and worms that supply power to both trucks... The wheelsets are blackened to look better. MR's sample started at only 2 volts and continued to run very smoothly at this low setting. The noise level hardly changed as the speed was gradually increased in the normal speed range to about a scale 90 MPH. Above that, things begin to whine, but few modelers will want to run it at the top speed... This model comes with truck-mounted Rapido-type couplers. This arrangement leaves a gaping hole in both pilots... All the production samples of these engines I have seen are beautifully decorated, with accurate colors and nice lettering... These are excellent models that have been on the N scalers "most wanted" list for a long time. I suspect they will be very popular. SD7: D&RGW, SP, B&O, B&LE, GN. SD9: PRR, Conrail, CB&Q, N&W, Southern. Price: $92.95")
SD24 reviewed: 07/04 Model Railroader ("This sweet-running, finely detailed ready-to-run N scale model of Electro-Motive Division's SD24 is made by Atlas Model Railroad Co. It's offered in both low and high nose versions with optional DCC decoders... The Atlas N scale SD24 closely matches the dimensions listed on EMD's specification sheets. The paint on our locomotive crisply applied. All the key SD24 features are present... The air brake piping along the roof is exceptional and very delicate... The model features a split metal frame that includes a recess at the rear to accommodate a decoder. The directional headlights are golden-white LEDs... Accumate couplers are body-mounted on both ends at the proper height. The SD24 performed well on the test track creeping along smoothly at about 6 mph. Road-switcher hoods don't leave much room for weight, but the locomotive managed .64 ounces of drawbar pull and wheeled a 15-car test train around 9.75" curves through no. 4 turnouts without a hitch. This is a great-running, great looking model of a standout prototype locomotive. ATSF, UP, CBQ, Southern, Undec. $99.95 ($134.95 with decoder)"