On the Schedules page, you can download PDFs of the schedules for specific routes. If you open the one for Toronto, Kingston, Ottawa, you can see all of the stations on that route:
- Port Hope
- Trenton Jct.
- Smiths Falls
You could then use the Stations page to get more details. (This page gets you the list, but not the map, you've asked for.) However the details for each station are quite poor - you can see where it is, and that's about it.
Now to keep going I need to know what you mean by "are accessible by public transport". Say that one of these small towns has a little bus route. Does that make that small town meet your needs? If so, great, the station page will tell you how to contact that local public transportation authority. Or do you need to get to them by public transport from somewhere specific, such as YYZ airport or Toronto's Union Station?
Assuming you want to reach the line from Toronto, you will want stations that are shared with GO, the commuter rail line. Toronto Union, Guildwood, and Oshawa qualify. (You can't see this on their web site, it is just a thing I know.) Alternatively, stations that are easy to reach by TTC - pretty much this means Union. If you want to reach Cobourg or Belleville by some other means such as a bus (Greyhound, Megabus, whoever is still in business), then you will have to drill a little further into the details about each station and set to work on the websites of those bus companies. I predict you will not enjoy that process.