What is a railway-share?

There is no doubt that it means a share in a railway company. Carroll's readers would have understood that straight away.

If you want to be on the safe side, then call the Snark "nonsense". That's the low risk approach to "The Hunting to the Snark".

However, Carroll himself stated that "The Hunting of the Snark" is about the search for happiness. Some (https://en.wikipedia.org/wiki/Railway_Mania) tried their luck by investing railway shares, e.g. Charles Darwin (http://darwin-online.org.uk/content/frameset?pageseq=103&itemID=A668&viewtype=text). Some earned a fortune, others were less lucky.

The word “snark” is, among other things, related closely to pigs - through “snork and “snorker” (English Dialect Dictionary, Vol.5, p. 572). This connection is strongly supported in Carroll’s poem by the fact that in it he makes a great many references to pigs. In 1870, almost six years before the publication of The Snark, a new railway line was opened between Garstang and Pilling, in Lancanshire, England. The line was later nicknamed The Pilling Pig, because:

"One of the earlier engines on the line had a particularly piercing whistle, which reminded some hearers of the sounds made by a pig in the process of being slaughtered." (Dave Richardson quoted in The Mail, March 13, 2019)

This “pig” was purchased in 1875. (https://en.wikipedia.org/wiki/Garstang_and_Knot-End_Railway)

Moreover, this line, better known as The Garstag & Knot End Railway, had ongoing insolvency problems due to the fact they could never sell enough railway-shares (https://en.wikipedia.org/wiki/Garstang_and_Knot-End_Railway), and:

"For most of its independent existence, the Garstang & Knott End Railway remained hopelessly insolvent and ordinary shareholders never received a penny of dividend interest on their shares. ("Dave Richardson quoted in The Mail, March 13, 2019)

Therefore these particular railway-shares were toxic for both the pig/Snark - by never raising enough cash to properly fund the railway line - and for the share-holders who never saw a return on their investments.