When the PV&T sold the class D’s to the Milwaukee Road, that wasn’t the end of dabbling in high horsepower units. Some of the more worn out class B1’s were being retired, so one of them – #220 – was pulled out of the deadline and sent to the Portland shops to be rebuilt.

What came out of the shops was the class B1x prototype; a 4-section articulated motor riding on 7 class B trucks:

weight

~750000 pounds

horsepower

7800 continuous / 5 minutes @ 10500

tractive effort

1062 kN

traction motors

4 @ 450 hp (end trucks)

10 @ 600 hp (middle trucks)

Initially, it turned out to be a too ridiculously powerful motor for the railroad; if not driven carefully, it ripped out drawbars without blinking and it was very good at finding lightly loaded cars in the middle of a train (by popping them off the track on a horseshoe or loop.).

It operated, first as a hill engine, then – and as larger and heavier freight cars came into service – as a general purpose road unit, for about 28 years (long enough for the class I (et seq) motors to come into service, so it was no longer considered a superpower unit before the end) before the talgo joint between the center sections failed, then was remanufactured into a class II motor with the same road number.