I could think of several reasons Private Cars wouldn't work. One the highest sleeping car number I think is 24 passengers most are lower. Each car has a different charter price some are as low as 1,000 a day some are upwards of 5,000 a day. If you want to charter coaches there aren't many single level coaches I can think of five off hat that are certified. Plus each car is different interior, and furnishings. So some people who get a coach ticket in say St Augustine will end up smelling that funky smell in the Exploded smerth. While someone who gets an IP coach gets something similar to a long distance coach on Amtrak.
Plus you would have to get a train manager to get everyone together I could think of three qualified people who could do that fairly easy.
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That is the locomotive guide for PVs with train length restrictions for the CZ.