The success of the M1 Mountain-type locomotives prompted the Pennsylvania RR to order 100 more locomotives of the 4-8-2 wheel arrangement in 1930, though several upgrades were incorporated in the new machines. The new locomotives became the M1a class and had a longer wheelbase than the M1’s due to the addition of the Worthington feedwater heater. They also received the new larger tenders of class 210F75, which became known as the coast-to-coast tenders.

The first 50 M1a locomotives - #6700 - 6749 were built by Baldwin; Juniata Shops, which became known as the “Altoona Works” built the next 25 M1a’s numbered 6750 – 6774, and the remaining 25 M1a’s were built by Lima and were numbered 6775 – 6799.

Starting in 1946, 41 of the M1a’s received further improvements by increasing the boiler pressure and the addition of circulators to the firebox. The improved M1a’s became the M1b class. The best way to tell them apart is to look for the circulators on the firebox, above the running boards - those round objects in a diagonal line that look like wash-out plugs.

The following locomotives became the M1b class:

6700, 6702, 6704, 6711, 6713, 6715, 6716, 6717, 6724, 6726, 6729, 6732, 6733, 6734, 6736, 6738, 6739, 6744, 6747, 6749, 6750, 6751, 6753, 6754, 6755, 6758, 6760, 6761, 6762, 6776, 6779, 6780, 6782, 6783, 6787, 6792, 6794, 6795, 6796, 6797 & 6799.

The PRR 4-8-2 ’s were considered to be the finest of the Pennsylvania Railroad’s steam locomotives. They were designed as dual service locomotives and were put to the test hauling passenger trains, general freight and in the end even the heavy ore trains, which they were not originally intended to pull. They did it all in style!

The factory installed Quantum sound system with dual speakers will feature the following:
• Chuff
• Whistle
• Bell
• Squealing brakes
• Doppler effect
• Sound of Power (change in chuff sounds with differing loads for more realism)
• Air let off (in neutral)
• Air pumps (in neutral)
• Water sounds (in neutral - blow down, pop off & injector)
• Blower hiss
• Headlight (dim head light in neutral)
• Slave (mutes whistle and bell for double heading)

Mechanical features will include:

• Powerful 5-pole can motor with flywheel
• Die-cast locomotive chassis
• Die-cast tender chassis
• Detailed tender underbody
• Plated bell & whistle
• Deck plate from cab to tender
• Many exceptional prototypical added-on details like piping and appliances
• Prototypical cab interior, including backhead, etc.
• Operating headlight
• Directional backup light
• Constant speed control - keeps speed constant up or down hill, etc.
• Authentic paint scheme and correct locomotive numbers
• Magnetic knuckle coupler on tender
• RP-25 contour on drivers and wheels
• Will operate on Code 70, 83 and 100 rail
• Can be operated on minimum 18" radius curves