in (one's) train
old-fashioned In pursuit of one; following closely behind one. They were forced to flee the country in their benefactor's train, facing persecution at home and having no resources of their own abroad.
in train
In motion or in progress; currently being prepared, processed, or dealt with. The article, which featured allegations from over a dozen ex-employees, set in train a series of lawsuits, firings, and investigations against business leaders accused of violating numerous tax codes. As construction is already in train, there is no point protesting against it.