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Peter's Model Railroading | The Layout | Equipment | PRR H21a
Construction: Frame


This is the start of a multi-page documentation of my scratch-building an S-scale PRR H21a quad hopper. The first one shown here has a number of errors in it and it isn't my best work. However, I have learned a lot from it, which will hopefully help with the future ones I am going to build. Most measurements mentioned in these pages are "scale", and you will be able to deduce from the context whether I am referring to actual dimensions (such as the thickness of styrene sheet used). That way if you are interested in following along in a different scale, you can easily translate the dimensions to your scale. My reference for this project was an article in the November 1978 issue of Model Railroader titled Scratchbuild a Wartime Hopper Car by the late John R. Porter. Although the magazine article is about building an S-scale hopper, it is only of limited use since the author describes building a generic two-bay World War I hopper. I was able to use it for building the frame of the hopper car.

Center Sill

The first step is to build the center sill. This is the back-bone of the car. I plan to fill the center sill with lead shot later on to add weight to the car. To build the center sill I cut a strip of 17" wide 0.020" styrene sheet, and attached two 2"x10" strips of styrene to form the basic U shape of the center sill. This makes the sill 21" wide. This photo shows the sill after the glue has set. The styrene sheet hasn't been cut to length yet, which needs to be done before the next step. The car is 42' from end sill to end sill, so the center sill is 42' also.

What I forgot to do was make some rectangular gaps in the top of the sill to insert the brake levers later on. I will remember to do that on the next car I build. I should have done that before I glued two 2"x4" strips of styrene to the bottom of the sill. The two strips create a lip on the outside of the center sill. The inside edge needs to be flush with the inside vertical walls of the sill. This takes a bit of practice and patience.

I then filed down two sections off the top of the sill which will take the truck bolsters. These 16" gaps were placed 50" from each end.

The bolsters start off with a 0.020" strip of styrene a scale 9'2" long (and 16" wide). These were then glued in the gaps, precisely centered and flush with the top of the sill. Make sure to also check for square, i.e. that they are perpendicular to the sill.

Next I glued 2"x2" strips to the underside of the bolster plate.

This photo summarizes two steps. First I cut a triangular piece of styrene 29" long and about 6" tall to fit on top of the 2x2 and under the lip of the center sill. The whole assembly was starting to warp a little, so I made sure to use weights to hold everything square as I installed the various bolster parts. That way the whole assembly is solid, straight, and square. The triangular pieces are visible on the left. On the right, I am adding the bolster bottom plates. The one on the right (already installed) should have been cut a bit longer.

Here's an extreme close-up of the finished bolster.

Side Sills

There are two side sills needed. I built mine from individual strips of styrene, before I discovered that Evergreen Scale Models makes "channels" that are the correct dimensions (part #266). The individual strips are 2"x8" and two 2"x4" pieces, glued together to form a channel that is 12" tall and 4" wide. Shown here is one assembled and other one in parts.

End Sills

The two end sills are a bit more complicated to build. I followed the above mentioned article exactly, and despite the complexity, they came out correct. (However, the end sills as-built don't match the look of the H21a's end sill. I will fix this in the next model.) All material used here is a scale 2 inches thick, except the square stock. Start off with two 8" wide strips cut to a length of 9'2". These represent the front (visible) part of the end sills. Next, I cut two 20" long strips of 8" material, which are the center spacers. These need to be whatever the width of your center sill is (i.e. its outer dimensions), because the end sill will eventually sit on top of the center sill. Next, I cut four 8" square tubes (I didn't have any solid styrene on hand) that are 26" long. Note that the overall width of the center spacer plus the two square tubes needs to be 6'. My center sill wound up being 21" wide, so I had to file out the opening later on to make it wide enough for the center sill.

After careful measuring and marking, I glued the center spacers on top of the front part exactly in the center.

The square tubes were then glued up against the center spacer. In the next photo you can see the top panel, made out of a 12" strip cut to 8'10" length, glued on top of the assembly. Note that the top panel is shorter than the assembly and must be centered across the assembly. The lip that is formed by the front and top panel is inserted into the side sill later on.

Now that the assembly is strong enough, remove the material from the front panel that matches the gap created by the center spacer and the two square tubes. This is seen on the top end sill in the next photo. The rear panel of the end sill is definitely the most challenging to make (these are the individual parts shown in the next photo). They are made out of 10" strips cut to a length of 9'2". Along the bottom of this panel, you need to cut a 1" strip off of each end that is 16" long. The end diagonal cross braces will snap into this eventually. Also note that in the opposite corners you need to make a 2"x2" cut-out. I usually measure and draw the outline to be cut, cut the critical dimension with a knife, and then file the remainder away until the material removed matches the lines drawn.

Once the rear panels are finished, they are glued to the third side (opposite the front panel, and under the lip of the top panel). Next, I cut and installed four bottom panels that measure 10" wide and 43" long. They are glued to the fourth side, against the lip of the rear panel, and separated by the center space. When those are installed, remove the center material of the rear panel, matching that of the front panel (not shown in the next photo).

Frame Assembly

We are now ready to assemble the frame. This photo shows the various sub-assemblies and their approximate positions.

The instructions in the above-mentioned magazine article have you only glue the end sills to the center sills, then build and install the end cross braces, and finally glue the side sills. I didn't think that that would yield a more-or-less guaranteed square frame. So, I decided to glue the basic frame and then deal with the cross braces later (which does make installing them a bit more challenging). The way I did my glue-up is I placed one end sill over the center sill, and positioned one of the side sills to one end of the end sill. I used a small T-square to make sure they were square, and then applied glue to the end sill-to-side sill joint only. Don't glue the center sill yet. I then did the same thing to the other end sill on the other side of the side sill. Next, I switched to the other side sill and glued each of its end sill joints. Finally, while holding everything square, I glued the end sills to the center sill. Again, make sure everything is square or else you wind up with a crooked car.

The corner braces are made up out of scale 8" wide Evergreen channels (part #264). Because of the fact that I glued the assembly together, they are a bit of a challenge to get into their positions, but not impossible. To prepare a piece, I held it over the area into which it is to be placed, and used a pencil to mark off the angles of the cuts. I marked it off just a bit longer, because the piece must rest on the lips of bolster and slide into the slot of the end sill. This photo shows one already installed and a second piece cut and ready to be glued in. You have to work at it a bit to get the piece to go in (slide it into the end sill first and then press it down into the corner of the bolster). Once inserted, glue it in place.

The side sills are somewhat weak right now, so a "midframe cross sill" needs to be made and installed. I made one following the directions of the magazine article, but that came out all wrong. I think the measurements in the drawing was incorrect. I decided to just make my own. I cut a strip of 2"x12" styrene to match the width of the frame, 9'8", made some cut-outs in the corner to clear the side sill (a height difference of 4"), and then made a center notch of 4" x 8" (I miscalculated the location of the notch, hence it is off-center here). After gluing the cross sill to the side sills and letting it dry, I then glued the sill to the center sill, making sure that the side sill were perfectly straight (use a ruler). By the way, the center notch is for clearing the brake lines.

The cross sill is actually composed of three parts. The second part is the vertical webbing continued from the top piece on down to where it rests on the lips of the center and side sills. Measure the part to fit, and cut out a small 2"x2" notch in the corner to clear the side sill. Note that you will need to put the assembly on something to raise it off the work surface due to the first part installed above. The photo below shows the second part installed on the left, and loose on the right.

The third part is the horizontal bottom of the cross sill. I cut it out of 2"x6" strips to the length to fit snugly in between the center sill and the lip of the side sill. Once the glue dries, this makes for a very strong frame.

The PRR H21a car has additional cross braces between the hopper doors. They are placed 14'8" in from the outside edge of the end sill. I made them out of 2" thick styrene cut and shaped to fit. They are angled down from the side sill to the center sill. I measured the length I needed and the height I needed, marked the angle between them, and then cut and filed the part to shape. You need a total of four of these parts. In the photo below, the left one is installed, and the right one is ready to be installed.

The next thing I did was to build up the section in the bolster area of the center sill, the part where the truck screw will be installed later. I cut various 16" square pieces of styrene until the thickness was built up to match the thickness of the center sill.

Before the brake gear is installed and while I still had full access, I glued in lead shot into the center sill to give the car some weight. Lead shot can be had at any gaming or hunting store. A 25lb bag will last you a lifetime! This completes the frame assembly.

Brake Gear (part 1)

Before building the main body, it is much easier to build the entire brake system onto the frame now. I decided to use 0.020" phosphor bronze wire (Tichy Train Group part #1103) for the piping of this car. I drilled a hole all the way from the end of the car through the bolster, with a drill bit slightly bigger than the wire to give myself some wiggling room. After inserting the wire through the holes from the inside of the car, I bent the wire (in place) to the shape you see in this next photo. I cut it off to match the distance to the car's center brace. I then did the same thing on the other side (make sure to do it on the opposite corner of the car!). When I was happy with them, I pushed them against the bolster as far as they went and trimmed them at the end of the car's end sill. I plan on inserting air brake valves (with glad hands) into the hole in the end sill, so the brake pipe wire doesn't have to go all the way to the end of the end sill.

I bought a set of the Westinghouse AB brake parts made by Grandt Line (part #4057). Using the parts identification sheet that comes with the set, and their approximate locations given the magazine article mentioned above, I glued them down to the top of the frame. This end becomes the car's "B" end (for "brake"). Before gluing the parts, I drilled out the holes on the parts where piping was going to be attached, using the same drill bit I used above. It was then just a matter of careful measuring, bending, and gluing of the piping. The brake parts were glued to small pieces of Evergreen "channel" strips. Some judicious filing was necessary to get the parts to fit.

Because I forgot to cut some gaps in the center sill (mentioned near the top of this page), I had to glue the brake levers to the underside of the center sill. This car is not meant for winning contests, so "close enough" is good enough. I glued a piece of wire between them, one wire each that terminates at the nearest bolster, and one wire on the "B" side that is to be connected to the parts above the frame later on.