This answer is incorrect.
"leave" is the base form of the
verb. It is also used for the present tense. This is an event that happened
before another event in the past. Therefore, the past perfect tense is
required. (In informal speech it is also correct to use the past tense.)
The correct answer is
The train derailed after it had
left the DeKalb Street station.
or
(The train derailed after it left
the DeKalb Street station.)