Anywhere between Trenton and Newcastle would be pretty much the same as far as train traffic goes since both the CN and CP Montreal-Toronto mainlines run parallel to each other along this stretch. Cobourg might have a slight edge since the CN local out of Belleville usually only works that far west and the CP local out of Toronto sometimes just goes that far east. Brighton might also be a good choice as there is a rail museum, Memory Junction, there. Port Hope, also, has long trestles carrying the railways over the Ganaraska River.
I guess it depends on what you prefer. Cobourg is a small-sized city with food and other amenities nearby while Newtonville is mostly farmland and forest. I think the Newtonville area offers the better photographic vantage points. One warning, due to city bylaws trains do not blow their horns going through Cobourg.