I dont recall there being a close up on the train tracks when Harrigan is walking in the subway tunnel
Anyway, it's most likely blood the Predator had on it's body as it ran back through the tunnels. Water in an underground area doesnt make more sense than blood dripping off the Predator.
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Seeing as you edited the original post. I don't see these "inconsistencies".
The scene is dark with strobe lights you cant see anything, the Predator most likely is pushing
some people out the way. The Predator wants the armed gang who were only in one carriage. The rest of the carriages had innocent people that Leona was ushering out. The more people that got out the carriage the more room the Predator had to move around.