Unlikely to be ballast in deep bore tube lines, Central, Piccadilly, Northern, Bakerloo etc.
These were built using cast iron sections, maybe concrete sections later, I suspect the tracks will be on a concrete slab with a sloped centre to allow drainage. Not sure what the station would be, probably something similar but maybe ballast due to lower speeds and less risk of lateral movement.
I've read somewhere, once upon a time, and a very long time ago, that ballast was not used on deep bore tubes due to the risk of the track moving and the stock becoming out of gauge.
Surface stock, or cut and cover like Circle and the Met used ballast through out tunnels and as far as I know still do, in fact I don't recall suicide pits on those lines but dimly recall some stations around the south eastern end are laid on a steel or concrete raft? possibly to do with the north embankment sewage tunnel that runs along that area. Can't recall if the sewage tunnel is the same depth or below the District/Circle lines in that area.