Through the Looking-Glass, near the beginning of Chapter 3. [Searchable text]
From the illustrated version [The Complete Illustrated Lewis Carroll, Wordsworth Editions, 1991, with an Introduction by A. Woollcott, illustrations by J. Tenniel] the guard looks like a train conductor, but was probably called a guard in the Victorian period. The guard is moving between the compartments on an outside rail, and so is actually shutting the window, and moving on to the next compartment. I think Alice is sharing her compartment with a Goat and a Beetle, or a gentleman dressed in white paper.
Thanks to Martin Vetterli for bringing this quote to our attention in his lovely web Wavelet Introduction (follow the link labeled "Introduction"; the quote is on slide 13).
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