The 4-12-2 represents the wheel arrangement of four leading wheels, twelve coupled driving wheels, and two trailing wheels. This arrangement was named the Union Pacific type, after the only railroad to use it, the Union Pacific Railroad. Only one type of locomotive with a 4-12-2 wheel arrangement was built: the Union Pacific Railroad's 9000-series locomotives, 88 of which were built by ALCO between 1926 and 1930. These locomotives were used to increase the speed of freight trains in flat country, and were fairly successful, but were maintenance nightmares, largely because of their use of an inside third cylinder driving the cranked second driving axle between the frames. There was no inside valve gear to worry about, however. ALCO had obtained permission to use the conjugated valve gear invented by Sir Nigel Gresley. This system used two hinged levers connected to the outer cylinder's valves to operate the inner cylinder's valve. The 9000 class locomotives were the largest to use Gresley gear.
In this front view of the same locomotive the third cylinder and the mechanism that controlled it can be seen below the smokebox.
Between 1934 and 1940 eight of the first fifteen locos had their Gresley gear removed and were converted to a "double Walschaerts" valve gear which utilized a double eccentric (return) crank and second link on the right side (similar to the gear Baldwin used on its 3-cylinder experimental compound 4-10-2 #60000), which operated the valve for the inside cylinder. Union Pacific referred to this system as the "third link." The 4-12-2's constructed from 1928 utilized roller bearings in the Gresley lever bearings, thus none of these engines were converted. The pre-1928 engines not converted received the roller bearing levers in 1940, and no further conversions were made.
During design the third and fourth driving axles were planned to be "blind" (flangeless) in order to improve curve handling, but ALCO's lateral motion devices on the first and sixth axles (which allowed the axles to slide up to two inches to the side) made this unnecessary. They had the longest rigid wheelbase in North America, and the longest in the world until the Soviet Union built their 4-14-4 locomotive in 1934. The trailing truck carried the same axle load as the drivers, which was unusual.
There has been debate as to whether the first driving axle of the 4-12-2 was cranked to provide clearance for the main rod connected to the second axle. Union Pacific drawings show no such crank on the first axle, and the Railway Age article says "The 67 in (170 cm) drivers permit the use of a straight axle on the front drivers..." The spacing between the first and second axles was increased by 18 in (46 cm) to provide clearance. Based on the published dimensions, this means at its closest the centerline of the inside rod was 11.645 in (29.58 cm) from the centerline of the first axle. (UP drawings reproduced in Kratville and Bush's "Union Pacific Type" books show the inside rod 113 in (290 cm) long and the first and second driver axles 88 in (220 cm) apart. The inside cylinder axis was inclined 9.5 degrees and was 32 in (81 cm) above the plane of the driving axles at a point 181 in (460 cm) ahead of the second driving axle, so the cylinder axis missed the centerline of the second axle by 1-11/16 inches. The rod centerline is closest to the axle when the crank is 54.49 degrees below horizontal.)
There is one surviving UP 4-12-2 "Union Pacific" type locomotive. It is number 9000 displayed at the Los Angeles County Fairplex in Pomona, CA
Whistles:
CV163=0 UP 844 "Steamboat" - Default
CV163=1 Union Pacific 3-chime steamboat
CV163=2 Steam Whistle 1
CV163=3 SP 4449 Daylight
CV163=4 DRG&W 5 Chime
CV163=5 SOO Line 1003
CV163=6 DL&W 5 Chime
CV163=7 WM 6 Chime
Bells:
CV164=0 Pnuematic Bell 1
CV164=1 Pnuematic Bell 2
CV164=2 Steam Loco Rope Pull Bell
CV164=3 SOO Line 1003 Bell
CV164=4 UP 844 Air Ringer Bell - Default
CV164=5 SP 4449 Bell
CV164=6 DRG&W K27 Bell
Dynamos:
CV165=0 Soo Line 1003 Dynamo
CV165=1 K27 463 Dynamo
CV165=2 SPTriple Dynamo
CV165=3 UP Dual Dynamo
CV165=4 Z27 Dynamo
CV165=5 T16.1 Dynamo
Air Pumps
CV166=0 Cross Compound #1 Soo Line 1003
CV166=1 Cross Compound #2 K27 463
CV166=2 Cross Compound #3 SP 4449
CV166=3 Cross Compound #4UP 844
CV166=4 Single Stage #1 Z27
CV166=5 Single Stage #2 T16.1
The ESU Full Throttle Steam files now include a few new logic features for added operational realism while still leaving you in control of HOW YOU wish to run your locomotive.
Heavy Load: F9 by default.
Similar to the “Drive Hold” button on the Full Throttle Diesel files “Heavy Load” allows you to adjust the “Steam Cut Off Valve” at any speed allowing for a fierce full chuff or drifting with snifters and rod clanks. Heavy load can also act as an offset allowing speed adjustments when engaged if desired.
Coast: F4 by default.
Opposite of “Heavy Load” Coast allows for a negative offset allow drifting sounds of Rods and snifter valves at any speed. You again have the option to hold the speed to adjust the speed with this offset active. Even allowing for an increase of speed with no chuffs as if drifting downhill.
Independent brake: F10 by default
Identical to the Full Throttle Diesel files. This allows for the locomotive to stop more quickly than its regular momentum would normally carry it. This one is pretty self-explanatory – just press the brake and come to a stop! For additional options there are 3 Braking rates that can be set up for user preference.
Articulation: Sound Slot 2
By adding Sound Slot 2 to the F8 Function Mapping and enabling the "Secondary Trigger" (CV250) you can add a second set of drivers making any ESU Steam File articulated.
Function Mappable Air Horn: F21 by default
Sound Slot 21.
Many Steam locomotives had a single chime airhorn equipped. Those modelling SP, MILW, and others can now have this feature and put it where they would like in the function mapping in addition to the whistle.
S0785 - Release Notes
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V-2 R-2:
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- updated independent brake
- updated boiler hiss
V-3 R-3:
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- updated steam chuffs sound slot
V-4 R-4:
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- changed whistle sound slot 3 maximum sound slot speed to 128