You seem to be assuming there would only be one main line, and that this is a proposed reduction. I don't see it that way at all. Assume the line is two tracks.

You seem to be assuming there would only be one main line, and that this is a proposed reduction. I don't see it that way at all. Assume the line is two tracks. Add two more at each station, for a total of four, but just at the stations. It's like with cars: we pull over to the side instead of just turning off the engine in the middle of the road. At a station that uses island platforms, from west to east you have: southbound through traffic, southbound stopping trains, the platform, northbound stopping trains, northbound through trains. At a station that uses separate platforms, from west to east you have: southbound platform, southbound stopping trains, southbound through trains, northbound through trains, northbound stopping trains, northbound platform. Of course, in reality the line may have sections with different numbers of tracks. It could be 1, 2, 3, or 4. There could even be gauntlet tracks.