Re: N4 cabin car
I should have been more explicit.
Yes, it is not a correct model out of the package. However the cast side frames appear to have the correct profile, and in this case they expect you to assemble the bolster and spring package to the cast side frames. You can fabricate your own bolster and leaf spring instead of having to hack up the plastic Bowser trucks which will be more difficult. Trying to keep the bolster openings and related geometry in the plastic side frames will be finicky and even minor differences on how you hold that shape will probably be noticable from looking at the model in a museum setting. You will have to do that four times too and then also fabricate a bolster and spring assembly for both trucks. If you mess up one modifying even one side frame you are already matching the cost of the brass truck as well considering shipping.
If you are active at all on the Facebook brass pages there is a gentleman who does his own casting of custom parts and I believe he offers it to other modelers. I can't say want the cost of that would be, although I doubt it would be much since it just seems like a pastime to him. In this route you would get a brass truck that can be soldered together, be strong and sturdy and have good weight to it as well. I prefer using metal trucks when I can. Even with good weight in the model you have probably found that trucks are light themselves may ride up on one axle only. This leaves the second axle with an opportunity to catch a turnout feature or the slightest mismatch in rail and take your car off the tracks. Frustrating.
Ironically it looks like this truck is made in O-scale, but I think it's labeled incorrectly:
Might be worth it to email them and ask if know of an exact copy in HO.
For what it's worth, I attack uneven surfaces of ACC and resin with a finer metal file like what Tamiya offers. The relative hardness is minor and the rigidness of the file prevents your tool from conforming to any unwanted profile on the workpiece.