Internally, these two models are identical. Externally, the main difference is the little removeable plastic side skirting pieces that cover the fuel tank on the H15-44. Beyond that, Atlas offers various prototype-dependant variations in shell detailing - body-mounted handrails, sill-mounted handrails, no handrails at all, square cab windows, round cab windows, etc.
These models have a really cool system of "classification lighting". In addition to the normal directional (white LED) head and tail lights, there are bi-color (greeen/red) LEDs on each end that illuminate markers on the shell (green on the forward side and red on the back end) -
The mechanism sports all the other features one normally associates with "modern" Atlas diesels - IE, split-frame / all-metal chassis, 5-pole / skew-wound "scale speed" motor, dual flywheels, low-friction drive, all-wheel drive and pickup (no traction tires), blackened / low-profile wheels, shell-mounted Accumate couplers, all-plastic gearing, etc. The chassis is fully DCC-Ready (and, in fact, available with a factory-installed decoder).
Performance on these models is perfect in every way - smooth, quiet, flawless pickup and throttle response, exceptional pulling power, etc.
A reported issue with at least one production run of these models relates to the tiny male universals (520104 / 540104) that make up part of the mechanism's drivetrain. Said plastic parts are press-fit inside the flywheels and have a tendancy to degrade/crack over time (to the point that they no longer spin when the flywheels spin). The solution is to order replacement parts from Atlas. Or, failing that, simply use some super glue to fix them more firmly in place.
Prototype information -
"In the late 1940s, the design team at Fairbanks-Morse (FM) set out to develop an 8-cylinder 1,500-horsepower all-purpose road switcher using their opposed-piston diesel engine design. By September 1947 FM had produced the first H15-44 locomotive in the form of Demonstrator “FM 1500”. The hood-unit style body included distinctive features such as slightly sloped ends, rounded corners, oval cab-side windows and a raised, rounded headlight base on each end.
"The first purchaser of the H15-44 was Monon, who quickly bought both the demonstrator and the second unit built in December 1947. Three units were then built for Rio Grande before additional demonstrators were produced later in 1948. Before production ended in mid-1949, a total of 35 units had been built, with Jersey Central owning the most units (14 total).
"By 1950 other builders had increased the horsepower of their competing road switchers, and FM responded with the 8-cylinder 1,600-horsepower H16-44 model. The earliest units were externally very similar to the previous H15-44. MKT 1591 was initially ordered as an H15-44, but was upgraded to 1,600hp during production.
"The first true production H16-44s were ordered by Jersey Central. Several large railroads purchased this model, including Santa Fe, New York Central, Southern and Pennsylvania. A total of 99 units would be produced before production of the “phase I” version ended in December 1952. FM continued to produce H16-44s with phase II and III body styles through 1963.
"Our model represents the “phase I” body style of both the H15-44 and H16-44 that was produced between 1947 and 1952. Options include FM or AAR truck side frames, high or low-mounted headlights, early or late-production phase 1 hoods and a removable fuel tank skirt."
To remove the shell, stick a couple of toothpicks (or whatever) between the shell and chassis and pry the sides out a bit. The shell should lift up and off quite readily at that point.
Reviewed: 06/03 Model Railroader ("Atlas has added a pair of excellent Fairbanks-Morse B-B road switchers to its N scale line. Both models have smooth running mechanisms and plastic bodies with railroad-specific details... Our samples closely match prototype drawings... The chassis is held in the body shell by four small detents above the trucks... The body is filled by the mechanism, weight, and a PC board containing the headlight and classification light circuitry. The class lights on the corners of the units change color between red and green with the direction of movement... The mechanism follows the common N scale split chassis design... The sample models started and ran smoothly at normal operating speeds below 6 volts... The drawbar pull is equivalent to about 12 cars... The wheels on our samples matched NMRA standards... Our sample came neatly decorated... all the painting is first rate with sharply printed lettering, including the number boards. These early FM road switchers are great additions to the hobby's fleet of N scale first-generation diesel locomotives. CG, CNJ, DRGW, KCS, RI, UP, ATSF, NYC, NYNH, Undec. $104.95 or $139.95 with decoder")