To better keep track of noticeable, and not so noticeable differences in appearance that a locomotive model would acquire during the course of its production run, locomotive historians began documenting any subtle or minor changes made to a particular diesel locomotive model as "phases", and referring to these as such. This practice has proved very popular over the years among diesel locomotive modelers looking to create the most "true to life" models possible. Despite not being official designations, phase descriptions are quite useful to the diesel spotter and record keeper, but sometimes tricky as many of the changes described are mostly cosmetic and easily altered features of a locomotive - roof fans, body panels, grilles, etc. that could be - and often were - updated or swapped interchangeably during production runs. The following are normally identified as F3 phases:  Built from July 1945. High, flat-topped 36 in (914 mm) roof fans. Top third body panel had "chicken wire" in openings only. Short rear vent panel. Center-third body panel with three equally-spaced porthole windows and D17 traction motors. As-built Phase I F3 units are identical to the F2, they differ only in electrical equipment and numberboard size. Three highly modified locomotives survive from this series, rebuilt as FP10s, all for Metro-North Railroad. Built from February 1947. Top third body panel now had full-length "chicken wire". Long rear vent panel. Center third body panel now had two portholes; area between covered with chicken wire, over 4 smaller rectangular openings. Built from December 1947. Roof radiator fans change to low, pancake fans. Built from March 1948. (Surviving F3's former BAR 42,44,46 are still Phase 2 and were built in MAY 1948) Center third body panel now has no chicken wire between the portholes; the four rectangular openings now have louvres. Built from August 1948. Chicken wire upper-third panel is replaced with full-length horizontal stainless steel grille. The first "F5A" EMDX demonstrator #59 was built in March 1948. Production of the "F5" started in August 1948 through the end of F3 production in February 1949. The difference between the "F5" and the F3 were the D27 traction motors with heavier-duty cables and higher capacity traction motor blowers fitted. Nearly all previously built F3's received the same upgrades by 1955. A total of 381 F5As and 238 F5Bs were produced.