Builder's photo of Altoona-built K5 No. 5698.

Pennsylvania Railroad K5 Class 4-6-2 Pacific Steam Locomotive
The Pennsylvania Railroad's class K5 was an experimental 4-6-2 "Pacific" type, built in 1929 to see if a larger Pacific than the standard K4s was worthwhile. Two prototypes were built, No. 5698 at the PRR's own Altoona Works, and No. 5699 by the Baldwin Locomotive Works. Although classified identically, the two locomotives differed in many aspects, as detailed below. They were both fitted with a much fatter boiler than the K4s, but with dimensions similar to those of the I1s 2-10-0 "Decapods". Most other dimensions were enlarged over the K4s as well; the exceptions being the 70 square feet (6.5 m2) grate area and the 80 in (2.032 m) drivers.

K4s and K5 In comparison:

Cylinders K4s: 27 in × 28 in (686 mm × 711 mm)

Cylinders K5: 27 in × 30 in (686 mm × 762 mm)

Boiler pressure K4s: 205 psi (1.41 MPa)

Boiler pressure K5: 250 psi (1.7 MPa)

Total heating surface K4s: 4,041 square feet (375.4 m2)

Total heating surface K5: 4,285 square feet (398.1 m2)

Superheating surface K4s: 943 square feet (87.6 m2)

Superheating surface K5: 1,634 square feet (151.8 m2)

Weight on drivers K4s: 201,830 lbs (91,550 kg)

Weight on drivers K5: 208,250 lbs (94,460 kg)

Total weight K4s: 308,890 lbs (140,110 kg)

Total weight K5: 327,560 lbs (148,580 kg)

Tractive effort K4s: 44,460 lbf (197.8 kN)

Tractive effort K5: 54,675 lbf (243.21 kN)

Factor of adhesion K4s: 4.54

Factor of adhesion K5: 3.80

The K5's factor of adhesion was much worse than the K4s'. This was due to the fact that the K5 was more powerful than the K4s, but with little more weight on drivers (and thus adhesion). Factors of adhesion below 4 are often considered undesirable for steam locomotives, and the K5 design did prove to be rather less sure-footed because of it. For this reason, 4-8-2 "Mountain" and 4-8-4 "Northern" designs with more drivers (and thus a greater allowable weight on drivers within the same axle load limit) were generally considered preferable for locomotives as powerful as the K5.

Both K5 locomotives were given a 130-P-75 tender carrying 12,475 gal (47,220 L; 10,388 imp gal) of water and 22 tons (20.0 t; 19.6 long tons) of coal. Surprisingly for such large locomotives built at such a late date, both were equipped for hand firing. Both were fitted with Worthington-pattern feedwater heaters, power reverse, un-flanged main drivers, and both used nickel steel boiler shells. As built, both carried their bell on the smokebox front, hung below the headlight; this arrangement was common on other roads but at the time unique on the PRR.

Altoona-built No. 5698
K5 No. 5698 was built at the PRR's Juniata Works (Juniata 4205 / 1929) in 1929, but having a works plate Altoona Works 4205 / 1929. This information is according to the late Mr. Ivan W. Saunders of Pittsburgh, PA. A one-piece cast steel locomotive bed was used; this produced the locomotive frame, cylinders, and smokebox saddle as one giant steel casting. This casting was produced by Commonwealth Steel's Granite City, Illinois plant. This cast locomotive bed design was a successful trial, and was repeated on 1930s order for 100 M1a class 4-8-2 "Mountains", as was the Worthington feedwater heater with mixing chamber behind the stack.

K5 No. 5698 used conventional Walschaerts valve gear and piston valves, and developed a starting tractive effort of 54,675 lbf (243.21 kN).

During the New York World’s Fair of 1939-40, K5 5698 was displayed along with several other PRR locomotives at the fair. In 1941, following the fair, 5698 was modified with a smokebox extension and a different feedwater heater within.

5698 was again modified by 1948 with a new (one-piece) smoke box, new cylinders with outside steam pipes, and “modern” front end.

Baldwin-built No. 5699
In contrast, Baldwin-built No. 5699 (Baldwin 60660 / 1928) used a conventional, fabricated locomotive frame, longer smokebox with the feedwater mixing chamber in front of the stack and the generator mounted on the right side (below the stack), and separate cylinder plus half-saddle castings. The innovation trialed on No. 5699 was its use of Caprotti valve gear, a poppet valve gear invented in Italy. The greater efficiencies of this gear gave the locomotive a higher tractive effort of 58,092 lbf (258.41 kN).

Although efficient, the Caprotti gear proved less reliable than the regular Walschaerts valve gear and required more frequent, more skilled maintenance, and in 1937 it was replaced by Walschaerts gear driving regular piston valves, making the two locomotives (except for frames) functionally identical.

Service life
The two locomotives were originally assigned to haul Philadelphia to Pittsburgh through trains. After the electrification reached Harrisburg, they were reassigned to the twisting Harrisburg–Baltimore route. Finally, they were reassigned to Pittsburgh–Crestline trains.

Both were updated after World War 2 with the standard front-end "beauty treatment" given to most K4s locomotives: a sheet-steel drop coupler pilot, higher-mounted headlight, and turbo-generator mounted on the smokebox front for easier access. They lost their unique bell placement at this time for a location equally unique for the PRR: under the generator-service platform on the front of the smokebox.

In Conclusion
Like many experimental locomotives, the K5 was only a qualified success. Both locomotives performed well enough to be kept in service, but they did not perform so outstandingly as to persuade the PRR to build more. They proved that building a larger Pacific than the K4s was not worthwhile; the reduced factor of adhesion limited the locomotives' ability to put their greater power to full use. A larger locomotive with eight driving wheels was needed.

This route was eventually taken, although in striking form with the T1 4-4-4-4 duplex locomotive, but the Great Depression and the huge cost of electrification meant that PRR steam locomotive development effectively ceased after the K5 and M1a. The electrical department became the thrust of PRR locomotive policy, and the displaced steam locomotives from electrified territory gave the PRR a surplus of locomotives, giving little incentive to develop more.

The cast steel locomotive bed and Worthington feedwater heater both proved themselves, and they became a feature of most PRR steam locomotives built after the K5, and indeed those on many other railroads.

Both locomotives were scrapped.

PRR K5 Overview
Type and origin
Power type: Steam
Builder: No. 5698: PRR Altoona Works
Builder: No. 5699: Baldwin Locomotive Works
Serial number Altoona: 4205
Serial number Baldwin: 60660
Build date: 1929
Total produced: 2
Rebuild date: 1937
Number rebuilt: 1
Specifications
Whyte Configuration: 4-6-2
Gauge: 4 ft 8 1⁄2 in (1,435 mm) standard gauge
Driver diameter: 80 in (2.032 m)
Adhesive weight: 208,250 lb (94,460 kg)
Locomotive weight: 327,560 lb (148,580 kg)
Tender type: 130-P-75
Fuel type: Coal
Fuel capacity: 22 tons (20.0 t; 19.6 long tons)
Water capacity: 12,475 gal (47,220 l; 10,388 imp gal)
Boiler pressure: 250 psi (1.7 MPa)
Heating surface: 4,285 sq ft (398.1 m2)
Superheater:
​ • Heating area: 1,634 sq ft (151.8 m2)
Cylinders: 2
Cylinder size: 27 in × 30 in (686 mm × 762 mm)
Valve gear No. 5698: Walschaerts
Valve gear No. 5699: Caprotti, rebuilt to Walschaerts
Performance figures
Tractive effort No. 5698: 54,675 lbf (243.21 kN)
Tractive effort No. 5699: 58,092 lbf (258.41 kN) as built
Factor of adhesion: 3.8
Career
Operator: Pennsylvania Railroad
Class: K5
Number in class: 2
Road numbers: 5698–5699
Date delivered: 1929
Disposition: Both scrapped

See also:

K4s Pacific

Steam Locomotives

PRR Locomotive Classifications

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Builder's photo of Altoona-built K5 No. 5698.
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