While the question in itself isn't that stupid, your attitude towards a thoughful and considered response was immensely stupid. But here it is with even more depth: No the Sandy line isn't the shortest in Melbourne, in fact here's some distances (all measured from Flinders St Station, stations counted not including city loop stations, or North Melbourne and Richmond) that I found in just a couple of minutes myself, using the same website linked above that you suggested was next to useless:
Williamstown 14.204km 9 stations
Alamein 16.12km 12 stations
Upfield 18.85km 13 stations
Sandringham 19.16km 12 stations
Comparisons to the former lines to St Kilda and Port Melbourne are irrelevant, they don't exist as heavy rail anymore for exactly that reason.
The difference between Brighton Beach and Sandringham is barely over 3km and contains only an extra 2 stations, so their is little to be gained by stopping a service short at Brighton Beach over just continuing on to Sandringham. Yes Brighton Beach does have a 3rd platform (platform 1 to be exact) but this hasn't been used for regular services for a long time, for the reasons above.
As for having a little respect for the OP, the OP was related to fixing bottlenecks, an issue that is not relevant to the Sandringham line.
The reason you should be expected to look up some of your own facts are due to the fact that you are the one seeking them, and they are available to anybody who can be bothered to look for them, proof is the above information that I managed to find quite easily. Why should we have to do it to prove your point for you?