I'll offer some answers, but this topic really belongs in the "Hi-Rail, 027 and Traditional 3-Rail Trains" sub-forum. Maybe a moderator can move it for you.
Way back when those cars were made, "high-end" still meant the same as it did in the 1950's--aluminum-bodied passenger cars 15 inches long. The length discrepancy you've noted is due to the inclusion of the couplers in that measurement.
These cars are not anywhere near scale length--they were made to negotiate the predominant curve trackage of the time--mostly 31-inch diameter. The cars would have to be 21 inches long to be proper scale length, but at the time, few had the kind of wide-radius curves needed to run those cars (most still don't). When scale-length models became popular, these 15" cars were demoted to the RailKing line, and then replaced with ABS plastic versions, most likely to be able to meet the price points of the reduced-cost line.
Most streamlined passenger cars were 85 feet in length, and and as proper scale length models, would measure 21.5" long, not including couplers. Even MTH's current Premier cars do not meet that criteria--they were shortened somewhat so that more people could run them on the intermediate curve diameters that became available between 031 (15" radius) and 072 (36" radius) in the meantime (home layouts having the curves necessary to host 21.5" cars are still very much in the minority).
To the best of my knowledge, no separate-sale interiors have been offered for these cars. A number of proposals have been floated by individuals to make interiors for empty-shell passenger cars like these, but none have yet to bear results. Same for diaphragms--they exist mostly as spare parts kept on hand by some of the companies offering them, but not offered for sale as upgrades for older cars even though it may be possible to buy small quantities of them via parts departments or resellers. The only folks who have equipped their diaphragm-less cars so far have made their own, in a fashion that naturally differs from the molded-rubber ones included on current passenger cars.
The "appropriate" locomotives for these cars would be any of the "covered wagon" diesels--F3's, Alco PA's or EMD E-series.
---PCJ
