It took how many years for Lynbrook to get its station? The suburb was to all intents and purposes completed, people had made their transport arrangements and settled into established travel patterns long before the long-promised station was built.
Similarly with Point Cook / Williams Landing. And South Morang though that did require reinstatement of the line as well.
It is typical of the long-term head-in-sand attitude of our politicians that these things are "planned" (i.e. someone's pet pipe dream) but don't eventuate until long after anyone who might make use of them has made (read "been forced to make") other arrangements to get about.
The line to Lilydale could be doubled. No doubt it will happen one day. The question is "Which day?" It would have to be doubled to accommodate any more trains than it now does and / or to cope with an extra 2 minutes each way in the schedule for an additional stop. No matter how strong the case is for building the new station.