The undertail is visible in the last photo - the dark centres to the feathers are an indication of this being an Eastern bird (subspecies indicus).
Other supporting features showing this to be indicus are the brown mask, paler supercilium, extensive brown on breast and relatively short deep-based bill.
Indicus is the regularly occurring subspecies of Water Rail in HK. There is one record of the western subspecies (either aquaticus or korejewi), at Mai Po in Dec 2006. There has been a suggestion that these are a separate species, so it worth checking any Water Rail you see.
