The Liberal government's pledge of 15-minute service across the GO Transit rail network is going to have a slow start, with many trains coming much less frequently at first, The Globe and Mail has learned.

Premier Kathleen Wynne and Transportation Minister Steven Del Duca are expected to announce on Friday details on the shift from the current commuter service, running largely in the morning and evening, to what the government calls Regional Express Rail: electrified GO trains operating all day, multiple times an hour.

For more than a year, the provincial government has touted 15-minute GO service. Last spring, Ms. Wynne promised to phase in all-day frequent electrified service. Around the same time, Mr. Del Duca's predecessor, Glen Murray, said the service would rival commuter rail in Paris, with electric trains running "all day in both directions every 15 minutes … across every corner of the [Greater Toronto and Hamilton Area]."

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Official documents and senior executives at Metrolinx, the transit agency for the Greater Toronto and Hamilton Area, were always more restrained than the public rhetoric, cautioning that not all lines had enough riders for such a service increase.

On Friday, the public is expected to start learning what changes are coming. According to sources, the move toward 15-minute service will begin only at peak times. In some places, trains will come every 30 or 60 minutes in the off-peak period. Not all lines will get the same increases.

Still, service will increase substantially. On many lines, trains now run only in the same direction as the commuting pattern, and not at all during the day. For example, Friday morning's political event is to happen in Barrie, and people wishing to come from Toronto will have no outbound train to get them there.

It is not clear how this increasing focus on regional rail affects the Big Move, a package of transit projects Metrolinx has been pushing for years. GO electrification is expected to cost billions of dollars, leaving an unknown amount for the other projects.

Toronto's proposed downtown relief subway line proposed for Toronto is mentioned in a Ministry of Finance backgrounder released on Thursday as one of a list projects for which "ongoing planning and design work will continue."

GO service increases will start on the Lakeshore lines, which carry about half the current ridership, between Burlington and Oshawa. It will also increase on parts of the Barrie, Kitchener and Stouffville lines. Toronto Mayor John Tory's SmartTrack transit plan relies on the electrification of the latter two lines.

Riders on the Milton and Richmond Hill lines – where freight company ownership of tracks will complicate electrification – will have to wait longer.

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Moving to a high-frequency service is an ambitious challenge, requiring everything from different rolling stock to new track. Stations will need to be renovated or built. The prevailing assumption that most GO passengers drive to the station and leave their vehicle there will have to be rethought.

However, the potential is also great. Running trains more frequently could turn the GO rail network into something between commuter rail and a proper subway. Although rider studies have not been released, the transit agency typically finds that additional trains put on the existing service fill up quickly.