The C1a Class were a type of 4-4-4

4-4-4

Under the Whyte notation for the classification of steam locomotives, 4-4-4 represents the wheel arrangement of four leading wheels on two axles, four powered and coupled driving wheels on two axles, and four trailing wheels on two axles. In the United States, this arrangement was named the...

 steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

 built for the Philadelphia and Reading Railway in 1915. Four locomotives were built, road numbers #110-113. They used front and rear trucks that were effectively identical. During a year of operation, they proved to be quite unstable; after that year, they were rebuilt to 4-4-2

4-4-2 (locomotive)

Under the Whyte notation for the classification of steam locomotives, 4-4-2 represents the wheel arrangement of four leading wheels on two axles , four powered and coupled driving wheels on two axles, and two trailing wheels on one axle...

"Atlantic" locomotives, reclassified as P7sa, and renumbered #350-353.