Standout features on both classes are the square-shouldered Belpaire firebox, the 72-inch driving wheels, and a large cross-mounted air tank positioned at the front of the locomotive. The differences between the M1a and M1b classes are minor. The M1b's have firebox circulators that are absent from the M1a's, and the sole visible distinction are the extra cleaning plugs on the firebox sides for maintaining the circulators.

In 2024, Broadway Limited is building models of the M1a and M1b. Offering three prototypical road numbers of each class, as well as an unlettered M1a, an unlettered M1b, and two PRR-style fantasy paint schemes that the M1's never wore while in service. All of the Paragon4 series models are equipped with our newest Paragon4 DC/DCC Sound decoder featuring Rolling Thunder and an integral GoPack capacitor pack.

• Paragon4 Sound & Control System featuring Rolling Thunder
• Integral Dual-Mode Decoder with Back EMF for Superb Slow Speed Operation in DC and DCC
• Precision Drive Mechanism Engineered for Heavy Towing and Smooth Slow Speed Operation
• Die Cast Body with Die Cast Chassis for Maximum Tractive Effort
• Premium Caliber Painting
• Prototypically Accurate Paint Schemes and Road Numbers
• Many Separately Applied Details such as Handrails, Grab Irons, Ladders, Piping, Whistle, and Bell
• Prototypically Accurate Sounds for PRR M1a Steam Locomotive
• Microtrains-compatible Couplers (2)
• Will operate on Code 55, Code 70, and Code 80 Rail
• Minimum Operating Radius: 9.75 inches
• Operates in DC and DCC with improved DC speed control (use DCMaster for DC Sound)
• Built-in extra capacitance to navigate imperfect track
• Pro Lighting Mode offers individual control of all lights on model
• Switcher Mode for precision low speed control
• Record & Play Operation - records and plays back sounds and movements once or repeatedly for automatic operation
• High resolution audio
• Quillable horn for various whistle lengths and patterns
• Choice of three selectable horns
• Alternate whistle/horn where applicable for locomotive with air horn and steam whistle - both the main whistle and alternate can be easily played
• Adjustable bell ringing interval for faster or slower bell
• Numerous user-mappable functions with available keys
• Multiple realistic passenger and crew sounds play on command
• Grade Crossing automatic signal
• Automatic forward / reverse signal
• Prime Mover sound intensity varies with load
• Individually adjustable sound volumes for each effect
• EZ Reset button for quick return to factory default settings