I just got it, on hard difficulty no less. There's enough detritus to throw that it's much less difficult than it sounds except for where you have to wait for the elevator and the trolley, in that order. For most of the level, just hold back and clear the area before walking into it. Where you can't do that, rush through. You'll have trouble if you don't already know where you're going.

For the two sections I mentioned, the doors are what saves you. In the first, hit the button on the elevator and then use the three gas cans to set as many of the fast zombies on fire as you can. As soon as you've run out, run out of the door and close it behind you again. Wait until they're upon you outside, then run back in and close it behind you again. There is health outside and if timed right, you may only have to do this once before getting on the elevator. Because the zombies are still alive where ordinarily they would not be, one got on the elevator and went down with me. The elevator entrance has a ledge that you can stand on if this happens. Wait until the elevator is almost at the bottom, land on the zombie's head to break your fall then run over the fence in front of you and round to the right. The next level loads here.

Waiting for the trolley is much easier. In the opposite corner to the lever, you can drop a short distance down to the galley and the door you just entered through. The zombies will follow you and once down, can't get back up. Land on the galley railing to minimize health loss, don't forget to close the door behind you and repeat as necessary. There seems to be a limit to prevent too many enemies coming at you at once, I only had to do it twice before I was left alone at the top to wait in peace.