I don't know why Russian is mentioned, that Boris the Brave guy is not Russian, but "Sleeper Service" was oddly translated in the Russian publication as "Спальный Состав", which means "Sleeping composition".

I haven't read the books, but I assume that the meaning of "состав" used here may be "train / a set of train cars".

That at least fits one level of the pun.

Hence “might”, “maybe”, “bit of stretch”