Arnold's S-2 was the first really serious attempt at a decent yard switcher for N scale. Up until 1991, the only other options were the somewhat primitive SW1500 "Cow & Calf" units (originally from Rivarossi, later from Kato) and Minitrix's weird little H12-44. And sure, the S-2's design is a bit dated by modern standards, but it was definitely state-of-the-art for the early 90's - both in terms of looks and performance. And despite the older design, these models remained popular for a couple of decades. Although admittedly, prior to Atlas's 2015 S-2 release, it's not like there were any other options.
Interestingly enough, this was the first N scale locomotive to come with a factory-installed decoder (or at least the first non-European prototype). The tiny dual-mode Lenz decoder is mounted inside the cab (pictured below), with wires soldered to the PC board for track power and motor control. Unfortunately, said decoder is a bit primitive by today's standards. Nevertheless, major kudos to Arnold for being the first manufacturer to bring DCC to North American N scale (and years before anyone else).
These models came packaged in the familiar Arnold "plastic box with orange paper inserts", and were primarily imported by Walthers. Rivarossi acquired Arnold in the mid-90's and newer units will be packaged with "Arnold-Rivarossi" inserts. Arnold's S-2 was ultimately discontinued with the bankruptcy (and subsequent liquidation) of the Rivarossi company in 2006.
This is a pretty complex model. And suffice it to say, getting at the guts of one is not an entirely straightforward operation (more on that in a moment) -
The chassis/frame is made of a lightweight zinc alloy and the cab is plastic, so most of the actual heft being provided by the all-metal hood piece. The motor is an open-sided 3-poler. The dual driveshafts are actually metal springs that turn the truck-tower gears directly (IE, no worms). The truck gearing is a mixture of brass and plastic. All wheels are geared and provide pickup. Sticky-uppy metal contacts on the trucks transfer current to a PC board that mounts inside the frame. The PC board in turn transfers current to the motor. Couplers are truck-mounted Rapidos (open pilots). There is no lighting. The wheel flanges are oversized, so these will not run on Code-55 track. The PC boards on the decoder-equipped models differ from the non-DCC models - the DC-PCB's route track current directly to the motor contacts, whereas the DCC-PCB's don't.
The vast majority of these did not come with traction tires. However (as pictured below) at least one production run did. I'm not sure when these came out, but my guess is that it was probably some time after Arnold was acquired by Rivarossi. I've never owned one myself, but I'm told by others that the tires do not interfere with pickup (don't ask me how).
Performance is pretty good, although not nearly up to the standards of more modern yard switchers (Life-Like's EMD SW9/1200, Kato's NW2, etc). Yes, pickup is good and pulling power is adequate. However, these models run fairly loudly and they have very little subtlety in terms of their throttle response (tending to start and stop on dime). Slow-speed creep is just OK. I think the main problem here is that "spring" drivetrain. Without flywheels (or even worms), there isn't much in the way of weight to smooth things out at the low-end of the throttle (or the high-end, for that matter). So, pretty good for the early 90's, but nothing to get excited about these days.
On the noise front, these locomotives have been plagued by two recurring issues (both of which contribute to the problem). First, if the spring/worms are anything but perfectly straight, they will wobble when they spins (with the end result being a lot of noise). So, when removing the motor, be very careful not to bend the spring/worms. Once bent, it's sometimes possible to re-straighten them by very carefully applying pressure in the opposite direction of the bend.
The second problem (a design flaw in the secondary gears in the trucks) is more serious. The gears pressed onto the axles (and the idler gears in the truck gearcase) have a dimple on them, resulting in a thin spot. As a consequence, the gears pressed onto the axles may crack at the dimple. You'll know if this is the case because an S-2 with cracked drive gears will make a clicking noise as it rides the rails. You can usually identify the axle with the bad gear by holding the locomotive in your hand and gently applying force to the wheels. The wheels should not move, but if one does, it probably has a cracked gear (which is now slipping). Northwest Short Line (nwsl.com) used to offer replacement gears and geared wheelsets for these models (not sure if they still do - but the part number for the gears is/was 2152-6, and the part numbers for the geared wheelsets are/were 2679-6 and 2680-6). Since this model is out of production, your only other replacement option would be to find a used one and cannibalize it for spare parts.
I list this locomotive as being DCC-ready in the index, but that should come with an asterisk. As mentioned above, a few (a very few) of these came with preinstalled decoders. Those that didn't come from the factory with a decoder already installed (IE, most of them) are not DCC-ready.
Trivia - In 2004, GHQ released a pewter shell kit for this model, allowing one to convert it into an Alco T-6 -
(Thanks for the picture, Ron)
As mentioned above, this is a pretty complicated model. Frankly, you can't really remove the shell without basically dismantling the whole thing. Handy diassembly instructions (complete with pictures) are included in the box. But if your instructions have gone missing, here's the Cliff Notes version -
1. Unclip the fuel tank
2. Slide the plastic tabs (that secure the trucks) towards the center of the frame (where the fuel tank was)
3. Remove the trucks
4. Remove the screw that holds the PC board to the frame
5. Pull the cab handrails out of the walkway (at the steps) so that the cab can be removed
6. Remove the cab by pressing in on the rear door and then sliding the cab upwards
7. Slide the hood backwards and then lift it up and off the body frame
Oh, and reassembly? Hey, have a ball...
Reviewed: 2/91 Model Railroading, 1/91 Model Railroader ("Yard switchers have always been a problem for N scale manufacturers due to the extremely limited space within the hood. There simply isn't much room to cram in a motor and the necessary mechanical parts. Even so, Arnold has taken up the challenge and produced a beautiful model of the S-2... Producing a model that runs well, with enough weight for reasonable tractive effort, took a lot of ingenuity. The hood is a highly detailed one-piece zinc-alloy casting. Molded openings within the hood contain the can motor and slippery plastic worm bearings at each end. The frame is another zinc-alloy casting that fits around a PC board floor. This board includes the necessary wiring and serves as a lock to hold the hood in place. The cab is a nicely detailed injection-molded styrene part. The trucks and handrails are made of a tough engineering plastic. All the wheels are metal and have a relatively large European-style flange. However, they seem to run through most ready-made turnouts without difficulty. All the axles are driven, and each of the wheels picks up electrical power. Our S-2 ran smoothly, though it's a bit noisier than some other recent engines. It is geared a bit lower than most N scale model, but I think that's fine as it provides a greater range of speed control in the normal switch engine speeds. With its eight-wheel electrical pickup, the S-2 travels across turnouts without hesitation... This model comes with Rapido-type couplers... Arnold's S-2 is a handsome addition to the roster of any N scale layout set in the post-World War II era. C&NW, CP, D&RGW, B&O, PRR, Seaboard, UP. Price: $89.98, $179.98 for a Digital control version")