Pennsylvania 2-8-8-2 "Chesapeake" Locomotives in the USA

Class HH1s (Locobase 325)

Data from Wiener (1930) and Roy V Wright, Locomotive Dictionary 1912 (Third) Edition (New York: Simmons-Boardman Company, 1912), Figure 107, p. 192. See also "Experimental Mallet Locomotive for the Pennsylvania," American Engineer, Volume 86, No. 3 (March 1912), pp.150-151; and May 1912, p. 265; and "Notes on Simple Mallet Locomotives", Railway Engineer, Volume 34, No 2 (February 1913), pp. 51-52. (Thanks to Chris Hohl for his 20 May 2017 email noting the correct tender coal capacity.) Works number was 50187 in September 1911.

A unique engine on the Pennsy, a simple-expansion articulated built at a time when all North American articulateds but one (Locobase 3570) were Mallet compounds. In a departure from Pennsylvania practice, this engine used a wagon-top boiler with the longest tubes of any engine in Locobase. A further departure was the builder--Alco didn't send many engines to the Pennsy. (But the next North American four-cylinder simple Mallet would also come from Alco in 1919 and would also be delivered to the Pennsylvania; see Locobase 309.)

Note the low boiler pressure, which was a particular requirement by the Pennsy staff. When they realized that low-pressure cylinders capable of generating the desired power would be too large to pass clearance limits, the railroad opted for four large simple expansion cylinders supplied by equally sizable 14" (356 mm) piston valves. Superheating the steam was thought to offset the loss of economy theoretically gained by powering LP cylinders with exhausting steam from the HP cylinders.

(Some sources show the superheater surface to have been 2,403.7 sq ft, (223.31 sq m), but Locobase has determined that the figure represents "equivalent superheater surface area", calculated as 1 1/2 times the actual superheater area to account for the higher quality of the steam. Locobase uses the actual value.)

AE report observed that four-cylinder operation meant carrying "a high pressure pipe from the superheater connection in the front end back to a swivel joint and then forward to the cylinders on the front engine." AE was quick to reassure readers that the setup had "been so carefully designed and constructed ...that it is believed that no trouble will be experienced with leakage."

Two months after its initial report, AE spotlighted a "splendid record run from Altoona to the mountain top" on 30 March 1912. 3396 pushed from the rear of an 1,125 ton (1,023 metric tons) train that climbed a stiff enough grade to generate a total resistance of 1,898 tons (1,725 metric tons) at 10 mph (16 kph). Two H8 Consolidations at the head end contributed about 700 tons (636 metric tons) of pull (If this is the train resistance overcome, that left about 1,250 tons for the HH1s.).

Fuel and water savings appear to work out as follows: The H8s consumed 4,500 US gallons (17,033 litres) of water and 6,000 lb (2,722 kg) of coal to overcome 36% of the resistance. The HH1s used 6,000 gallons (22,710 litres) of water and 9,000 lb (4,082 kg) of coal to handle the other 64%. So, if Locobase's inference is correct, the HH1s consumed about 57% of the water used by the three locomotives to conquer 64% of the resistance and 60% of the coal while contributing about half of the combined adhesive weight of the trio. Regarding the latter, the AE account said that the "mechanical stoker on the Mallet proved to be 90 percent efficient."

Two observations about the statistics above: 1) The H8 as delivered with a saturated boiler (Locobase 1032) and its superheated update (Locobase 5492) would have yielded different results. The AE report doesn't give road numbers for either H8 in the hill climb and thus Locobase can't identify the variety of apple being compared to the pumpkin that was the HH1s. 2) The expenditure of water and fuel for a single trip "to the top of the mountain" underscores the inherent inefficiency of an open power system using steam without condensing the exhaust. Although many inventors tried different ways to reuse exhaust steam, only compounding and exhaust steam feed-water heaters might be said to have recycled any of the remaining "push" in steam as it left the cylinders.

As Locobases 67, 5153, and 15921 show, the PRR found its superheavyweight freight hauler answer in the I1 series of 2-10-0s that premiered in 1916. Their boilers were worked hard, but contained much higher percentages of superheater area in their combined heating surface totals and their boilers were pressed a full 90 psi (6.2 bar) higher. They also had taller drivers.

3396 operated in pusher service until it was scrapped in April 1928.