Well, the concept art of underground shows it as an actual medbay, but I always assumed most of those props were cut for optimization purposes.
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Even in the game itself though, it’s clear that Underground was being used for the storage and transport of dead bodies. Of course they were probably transporting other things, but bodies are what we see most. Because of this, a medbay would make sense.
However, what doesn’t make sense is that destroying the fan controls, which also happen to be located in the medbay, somehow clears the gas from the lower levels of underground. You think the CDA would clear it anyways so they could move cargo, but here’s my theory:
The CDA are informed of Jackal’s plans to attack the london underground to destroy their supply route (which had previously been used to treat people of radiation sickness and store bodies for transport of those less fortunate, hence the medbay). They use the medbay’s fan controls to flood the underground with toxic gas, so Jackal can’t reach their target.
However, jackal figures out that destroying the controls activates a safety feature that turns the fans back on, allowing them to safely reach their target.