These trains used to run from Dadar for a very long time. People are used to calling it by that name.
There's nothing wrong in using a common name for a train. They are used by people familiar with the train, so it's not a problem.
Besides, only people from Bombay will identify that Dadar and Kurla are not the same. For all outsiders, the name doesn't matter. They just know that the train goes to Bombay and that's all they care about.