PRR M1

Pennsylvania Railroad M1

PRR M1a on display at the 1939 World's Fair. Pipe on the smokebox and box behind stack are components of the feedwater heater.

Type and origin Power type Steam Builder PRR Altoona Works, Baldwin Locomotive Works, Lima Locomotive Works (50) Serial number Lima 7099–7123, 7443–7467 Total produced 301

Specifications Configuration 4-8-2 Gauge 4 ft 8 1⁄2 in (1,435 mm) Driver dia. 72 in (1.829 m) Adhesive weight 271,000 lb (123,000 kg; 123 t) Loco weight 390,000 lb (177,000 kg; 177 t) Water cap 22,090 US gal (83,600 l; 18,390 imp gal) Tender cap 31.5 lb (14.3 kg; 0.0143 t) Boiler pressure M1a: 250 psi (1.72 MPa), M1b: 270 psi (1.86 MPa) Cylinders Two Cylinder size 27 in × 30 in (686 mm × 762 mm)

Performance figures Tractive effort M1a: 64,550 lbf (287.1 kN), M1b: 69,700 lbf (310.0 kN) Factor of adh. M1a: 4.20, M1b: 3.89

Career Operators Pennsylvania Railroad Class M1, M1a, Number in class M1: 201, M1a: 100 Numbers M1: 6699, 6800–6999, M1a: 6700–6799 Disposition #6755 preserved, remainder scrapped

M1 is a class of steam locomotive of the Pennsylvania Railroad (PRR). It was a class of heavy mixed-traffic locomotives of the 4-8-2 "Mountain" arrangement, which uses four pairs of driving wheels with a four-wheel guiding truck in front for stability at speed and a two-wheel trailing truck to support the large firebox needed for sustained power. Although built for both passenger and freight work, they spent most of their service lives hauling heavy high-speed freight trains. Many PRR men counted the M1 class locomotives as the best steam locomotives the railroad ever owned.

A single prototype, #6699, was built in 1923 at the railroad's Altoona Works. It spent three years in testing, including all kinds of main line service as well as a session on the railroad's static test plant. Having received favorable reports, a further 200 were ordered from commercial locomotive builders; 175 from the Baldwin Locomotive Works and 25 from Lima Locomotive Works. Breaking with the PRR tradition of random locomotive numbering, these were numbered in a solid block from #6800-#6999.

In 1930 100 more were ordered; this class M1a had several improvements. Instead of separate cylinder block and smokebox saddle castings, the M1a had a one-piece casting (first seen on the K5 Pacific) with inside steam delivery pipes, instead of the outside, visible pipes of the M1. A Worthington feedwater heater was installed, with a boxlike mixing chamber behind the stack. Twin cross-compound air compressors were hung next to each other on the left side, instead of the single compressor of the M1. The M1a locomotives had larger tenders than the previous locomotives.

The M1a locomotives were intended for passenger as well as freight service, and some bore decorative gold-leaf lining on tender and cab sides, but they proved better suited to freight work, and extensive electrification saw a surplus of K4s locomotives available for passenger trains. While they were mostly used on freight, their dual service purpose entitled them to have smokebox mounted keystone shaped numberplates. Freight engines on the Pennsy had circular numberplates.