Most of the Combine's trains run aboveground, some of them utilizing the old light rail network. Before the light rail, though, there was the city subway- narrow tunnels just barely wide enough to keep two trains running in opposite directions. The nearest passage into such a place lies directly in front of what used to be a respectable business building, a bakery by the look of things. It's mostly wrecked now.

It's also guarded by a couple of Civil Protection officers. They're masked, of course, but their postures suggest a general desire to be somewhere else- anywhere else.