The 3100 class GP-20, ex-3100 class, represents the few components left of the FT's. Those FT's were traded in on GP-20's, but some of the later models and their components were used in the newer engines. Any layout modeling an era of the late 1960's or after could easily include a GP-20 of the 3100 class; these would sport the characteristic yellow war bonnet and silver trucks. Previous to this, however, the GP-20 would appear with the older numbering system and the regular blue, non-warbonnet, paint scheme. These and other modifications will be discussed in this article.
The GP-20's, as delivered, were numbered 1100-1174. They were renumbered in the late 1960's and early 70's to 3100-3174. As of March 1975, numbers 3105, 3130, 3152, and 3160 were taken off the roster, apparent victims of wrecks. The use of the GP-20 was, and is today, multiple and varied. They have been found in yard service in places such as Way, Wichita, Belen along with many others, as well as being used in road and local yard service between Argentine and Corwith for a period of time. They are used in local freight service, as mainline freights, and have been mu'd with almost every type of hood unit. If mu'd with CF-7's though, the GP20 must be in the lead in order for the sanders to work. It is a very versatile engine and may even be run as a single unit on a branch line local.
Modeling the GP-20 in HO presents no particular problems (don't try modifying an FT unit as there is an easier way), however, there are no known units available in the N-gauge market. All detailing will be the same, regardless of scale, with the single exception of the dynamic brake information.
A major change must be made with the Tyco GP-20 by adding a dynamic brake available from any Athearn GP-7. This works out fine as the ATSF GP-7 does not have dynamic brakes. The Athearn model comes with two removable sides and by using a sharp knife or razor saw, the fan portion of the body top can be removed. The three brake pieces can then be put together and glued to the Tyco unit.
For the next step, cut a 3/32" length of plastic tubing of about 3/32" in diameter. This is cemented to the proper spot for the radio antenna on the center front of the cab roof. (See photos and illustrations accompanying this article.)
Warning flasher lights can be made of scrap pieces of plastic about 1/8" in diameter, not to exceed 5/32". With a mill file, round a ball on top with a slight indentation around the bottom. A small base under the ball with a flat bottom should be left so the three legs may be attached in a triangular fashion. The legs can be made of thin strips of scrap plastic and should be about 1/16" in length. The overall height should be from 3/16" to 1/4" and although this is actually oversized, any smaller would be very difficult to work with.
The horns on the model need to be moved and it is necessary to cut them from the cab roof. File the old mounts smooth and fill in any holes left; file smooth after dry. The horns should be located in back of the cab roof on the firearms side of the long hood. Using a piece of scrap plastic to make a mount bracket approximately 1/16" x 1/4 inch, make two scallop notches in the top for mounting the horns. Cut the legs off the Tyco horns and mount as in the illustration.
Check photos of the GP-20 radiator fans as some need extensions and some do not. The Tyco model has built-up fans unlike the ATSF contour and it is possible to file them around the outside or file them down for the low fan shroud. Painting the model can be accomplished by masking off and spraying. If brush painting the yellow warbonnet, a photo of the engine should be used as a guide. With a pencil, outline the contour where the bonnet should go and cut a shallow line along the outline with a sharp knife. Using either Scale Coat or Floquil's reefer yellow, brush the paint on so it will flow to the knife line, but take care not to use too much paint or brush beyond that line. Paint all handrails, the radio antenna, warning lights and horns reefer yellow at the same time. The warning light should have a silver base below the bulb and above the legs. To paint out the yellow stripe at the top along both sides of the long hood, use Scale Coat Santa Fe diesel blue. Paint the truck wheels and side frames silver.
To decal, I suggest using Microscale decals. A special ATSF warbonnet set is available with a straight nose decal. The wings of the emblem should go back to the cab, but unfortunately, the decal is not quite long enough. This problem is solved by cutting the wings off another nose decal emblem and extending the wings back to the cab. Use the blue numbers under the side cab windows on the yellow war bonnet. Finally, number the front and rear number boards. When finished, this should yield a modern-looking unit completed with only a few needed details in a minimum amount of time.