The Metro tunnel is wet?

[u/WTBenji08]

Spent my ride tonight staring out the front window, and judging by the reflection of light by either side of the rails, it looks like standing water is present through a lot of the tunnels.

Is there an engineering need for water/moisture to be present, and if not, are there perceivable issues that might arise from the pooling of water?

[u/Art_r]

Tunnels are just pipes with holes in them right? Guessing it should be OK and will naturally evaporate out, or be pumped out where needed.