Alkan's Op. 57 consists of three 'Chants' (Songs), not nocturnes, and 'Le chemin de fer' is actually Op. 27. These works are known for their extreme technical demands and unique harmonic language.
The sheer physical stamina required to execute the rapid octave passages and dense chordal writing at tempo is the primary hurdle.
2 Nocturnes (Op.57) is rated Henle level 9 of 9 — virtuoso repertoire, roughly concert / virtuoso level.
You should be comfortable at Henle level 9 — meaning you can already play level 8–9 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of 2 Nocturnes (Op.57) lasts about 10 minutes.
2 Nocturnes (Op.57) is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.