I know the end platforms of UTLX X-3, X-4, and X-5 tank cars were filled in between the bolster and end beam, but what were they filled in with?

Scott Chatfield

There is an 8-page document on prototype detail written by Steve Hile and posted on the Resin Car Works site. This shows several variations in the platforms with prototype drawings.
http://www.resincarworks.com/extras/kit4-01_UTLX10kX-3_protodata.pdf

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Rufus Cone
Bozeman, MT