The M1a's and M1b's are 4-8-2 "mountain" type steam locomotives operated by the Pennsylvania Railroad.  Use primarily for hauling freight, many PRR crews regarded the M1's as the pinnacle of the railroad's steam power.  

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, BLI are building models of the M1a and M1b.  BLI are 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 BLI's newest Paragon4 DC/DCC Sound decoder featuring Rolling Thunder and an integral GoPack capacitor pack.   In addition to the Paragon4 Series models, BLI are also offering these "Mountains" in BLI's Stealth Series.  Stealth Series models are No-Sound / DCC-Ready models that operate on DC out-of-the-box, or a modeler can easily install their DCC decoder of choice.  Out of the box, front and rear lights are directional.  Other lights are always ON with track power.  

Industry-Leading Model Features:

• Stealth Series - Model is DCC-Ready out of the box.  No Sound
  
• Out-of-the-box, front and rear lights are directional.  Other lights are ON with track power.
• Model comes with 8 ohm speaker installed for easy installation of a DCC/Sound decoder.
• 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
• Microtrains-compatible Couplers (2)
  
• Will operate on Code 55, Code 70, and Code 80 Rail
• Minimum Operating Radius:  9.75 inches