A grade crossing accident in which the crew were killed led to the substitution of a central cab to give better crash protection, a streamlined steeple type, in later production, a design which was also applied to the GG1.

Orders were placed for 90 production locomotives classified P5a due to minor changes from the prototypes (notably, larger traction motor blowers).

A fatal grade crossing accident on the New York Division confirmed traincrews' concerns about safety when the crew were killed after colliding with a truck loaded with apples.

[1] Documentation published in 2010 disproved the decades long belief that the modified P5's new shell design came first and was then applied to the GG1, R1, and eventually the DD2.

Visually, the class P5b could be distinguished from a boxcab P5a by having a lower row of ventilation grilles on the sides of the superstructure, and by having outside brake cylinders on the trucks.