Maryland Thunder is Western Maryland #1309, a massive 2-6-6-2 articulated compound Mallet steam locomotive built in November 1949 by the Baldwin Locomotive Works. It holds the title of the largest regularly operating compound steam locomotive in the world and was the final steam engine erected by Baldwin for the domestic US market. Weighing 434,900 pounds (with a total weight of 643,100 pounds including its tender), it spans a length of 99.7 feet and stands 15 feet tall.

Its appearance is defined by an intricate, raw mechanical anatomy. The locomotive features a long, matte-black steel boiler operating at 210 PSI, flanked by high-mounted sand domes and steam domes. Mounted prominently on the front of its smokebox door are two massive, cross-compound air compressors—a signature "C&O look" that gives the front end an imposing, muscular silhouette. Beneath the boiler rests its articulated 2-6-6-2 wheel arrangement, consisting of a 2-wheel leading truck, two separate sets of six 56-inch driving wheels (each driven by its own massive set of pistons, crossheads, and main rods), and a 4-wheel trailing truck. The front engine set pivots independently on a hinge to navigate tight mountain curves, featuring smaller, high-pressure cylinders at the rear and enormous, low-pressure cylinders at the front to reuse steam. The boiler is fed coal via an automatic mechanical stoker from a giant, high-capacity tender at the rear, stenciled with the iconic Western Maryland speed lettering. It vents towering columns of pitch-black coal smoke from its short smokestack, blasts dense plumes of superheated white steam from its cylinder cocks, and is topped with a distinct, screaming steam whistle. It is a literal, non-human, non-anthropomorphic piece of heavy industrial machinery that functions purely on coal, water, and raw thermodynamic force