[Bigtime]

I asked this question over at the Calgary Public Transit thread at skyscraperpage and the forumer Frinkprof offers this explanation:

As I understand it, they do. The trouble is that ends of games are difficult to predict and a last-second goal or a brawl at the end of the game or something can prolong things 20+ minutes. The extra trains would be coming from either 10th Street (southbound) or Haysboro or Anderson (northbound). So from the final (for sure) buzzer, there would be a time delay for the extra trains to get there, which may or may not coincide well with trains on regular operation, as well as the traffic bulge on the platform. That in itself is difficult to predict as people will stay until the end of a close game, while some will leave with a couple minutes left of a game well in-hand (or out).

The 'C' Platform is barely ever used as it was originally, that is to board on that extra platform, go northbound, do a "short turn" at Olympic Plaza/City Hall (operator gets out and switches ends) and then heads up to the NE. As you say though, they sometimes use the siding for an extra train for large events, where it backs out and approaches the main platform to come into service.