One method is for selecting the most recent rows is:
select t.*
from releng_retry_test_phases t
where t.date = (select max(t2.date) from releng_retry_test_phases t2 where t2.train = t.train);
If you actually want to modify the table and delete the older rows;
delete t
from releng_retry_test_phases t join
(select t2.train, max(date) as max_date
from releng_retry_test_phases t2
group by t2.train
) t2
using (train)
where t.date < t2.max_date;
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