GO Transit added limited mid-day and evening service on this corridor in October 1990. three mid-day trips Union to Erindale and one roundtrip to Milton in the evening.
The service was summarily dialed back 2 mid-day trips just one year later in October '91, this was one round of GO cuts imposed under the Bob Rae NDP gov't that also cut service back on other corridors as well, for instance they cut service to Acton and Guelph and rolled that corridor back to Georgetown in July '93
Mike Harris weilded the Axe again in '96 cutting the remaining mid-day and evening service.
For a history of significant GO Transit Milestones, good and bad, see this site:
http://www.trainweb.org/railwayop/significantdates.htm