If I have a single sink in my kitchen [which means that it is trief] can I place a rack (different ones for milchig and flieshig) on the floor of the sink and 1] wash with hot water. 2] drain a boiling pot into the sink without making the racks trief?
Stainless steel sinks can be kashered by cleaning thoroughly, and pouring boiling water. Enamel Porcelain sinks cannot be kashered.
For a sink that is not kosher, you can use the racks with hot water, and likewise drain a pot into the sink.
[There is a preference of using a full basin insert, based on the ruling of the Minchas Yitzchak.]
Between using the sink for meaty and dairy, ensure that the sink has been cleaned.
See Iggros Moshe, Yoreh De’ah 1:42, who is generally fairly lenient concerning sinks, based (among other factors) on the fact that irui (pouring) cannot affect two transfers of taste. See also Minchas Yitzchak 2:100, who takes a more stringent line.