Hello Piper Classique,
The Trans Canada train will afford you magnificent scenery. From the list of destinations for Via, the rail operator, the following stops are places where float flying may be available close by:
British Columbia - Vancouver city proper, at best you'll get a commercial ride, I don't think there's any floatplane rental/instruction there. East of Vancouver, Pitt Meadows airport is a busy float plane base, but it's not a stop for the train, and the better part of an hour car ride from downtown Vancouver. Hope and Chilliwack are stops and possibilities. Kamloops will have some float flying. Don't expect any convenient [to the train] float flying in Alberta nor Saskatchewan.
In Manitoba - An hour's drive north of Winnipeg is St. Andrews and Selkirk. There's float flying in Selkirk, though prior arrangements should be made. Harv's Air at St Andrews airport will know.
In Ontario, Sioux Lookout, Temagami, Sudbury, Parry Sound (a certainty), and Ottawa (Rockcliffe Airport) are stops on the route at which float flying should be available.
In Quebec and the Atlantic, you'll be on the "South Shore" which has few float flying operations. The Quebec north is where the float flying will be.
There are certainly other float flying centers which can easily accommodate you, but they are not train stops. Orillia is one, though their operation is paralleled with the operation in Parry Sound. Float flying around Parry Sound and Georgian Bay will afford you magnificent scenery.
I hope that helps....