Go on the map "Hail" when playing horde then run to the train cars with no doors.

Stay in there and everyone will come for you.

There is no way in (except for jumping over the walls) so boomers, grinders, maulers, flame boomers, sires, tickers, kantuses etc cannot hurt you.

Make sure you have a lancer or shotgun on you though because locust will jump over, as will wretches.