I have my list of the 4 best items to have blessed model freight car building as follows, in no particular order:
1) Evergreen styrene strips
2) Dial calipers
3) Resins, such as polyurethane
4) RPM meets and online information
Talking about resin-- the ability to take something as viscous as motor oil and pour into a mold cavity surrounded by rubber and get a solid Westerfield part is pure great science! The ability to show detail to amazingly small and thin sizes is remarkable, as well. This ability of going down to really thin sizes is both remarkable and a bane. All flash on a resin part is what is left behind when the casting process's smooth backing for the casting gets wet, which it must. The wetness will harden, as resins will and after the removal from the mold the flash on the part is a remnant of this wetted back side of the smoothing backing tool (this is what makes for the really smooth backing to flat cast parts). Though removing flash with a knife is often the technique used, it does leave behind a slightly raised portion which left on a car side, no harm because that portion of casting is on the back and inside of the part. The ends, however, rely on its flat finish to cap the pair of car sides and with flash a bit thicker than average this will cause a relic viewable between the end's edge and the car side. To eliminate this issue, sanding the end casting on a plate of glass until the flash is gone and the end's edge is straight is the preferred way to prepare these end castings for assembly.
This flash can, at times, work to our advantage. Not often in resin car building do we face the engineering problems of scale thickness. A good example is for running board edges. A wood running board is about 0.020" thick in HO scale. Fortunately, we don't need to model Dreadnaught ends to scale thickness, as the inside view, particularly the inside edge view is buried inside the car body. An issue is when we come to Dreadnaught ends which are viewable from both sides, such as a gondola end. The thickness of a 5/16" thick piece of stamped steel in the real world equates to under 0.004" in HO scale. That is about double the thickness of human hair and a real challenge to injection mold a cross section that thin in HO.
With the understanding of how thin wet resin can get gives us an opportunity to use that to our advantage. I have an example of how thin we can get with resin. An example is with a recently cast Dreadnaught end I have for a gondola drop end. My 2 mold pieces can be closed up to each other to allow very thin gaps. This end has about 0.004" thickness, and any attempt to get to even thinner cross sections is unnecessary at this reached closeness.
So here is a picture of a resin HO end which is probably thinner than any commercial plastic car's end made.
Outside view with the ribs facing outwards.
Inside view with the ribs going inwards.
You all do well,