The volume level of the white noise must be higher than the volume of the sound being masked. If the train is loud this would sound like Niagara Falls from the bottom. White noise works well for masking low level conversations .
In the telephone industry in analoge days, white noise was intentionally generated on circuits to mask crosstalk. People would put up with a swishing sound but they would get upset hearing conversation ,because they would deduct that others could hear them as well.