Ron Merrick wrote:
"Hope somebody will humor me, but I'm interested in the X40 cars despite the fact that I'll never build a model of one, one reason being that I doubt that one of them ever visited my obscure secondary main line in Kansas. I believe the X40c was the fire engine delivery car, which has been discussed here several times."
While the single Class X40C car was a 60 ft non-rack equipped automobile boxcar with end doors that could have been used for delivering fire engines, it wasn't necessarily dedicated in that service, so could be used for any other lading requiring an end door automobile boxcar.
Regarding the dedicated car for American LaFrance, you're probably thinking about the Class X30 boxcar.
"But I'd be curious as to what the others were intended for, other than pushing the envelope on freight car design. I see by the prr.railfan.net site that they had a fairly long life, but since only 110 were ever built, they may have been an evolutionary dead end."
They were used in automobile service, with a few equipped for LCL service - a check of the notes section of the ORER would indicate which ones were equipped with racks for specific parts and other service.