One of these would probably work. Or could could cover the whole window with plastic like this. Personally, I've got the rubber weatherstripping around my front door and one of my windows.

"could could" = "you could"

I don't know. I always did the sheets of plastic, to create a double-paned window-like seal.

and if all else fails: bean snakes

almost always a good winter gift and fun/cheap/easy to make

Hmm, I'll try a beanie snake. I want to keep the view clear but I may fold on this. Thanks, all.

Not sure exactly what you mean. You don't want the weatherstrip in the track, after all, you want it to seal between the sashes.

The actual track should have some sort of built-in rubber or vinyl or metal springy seal thing. If it isn't working well, you'll want to replace it, not add in something else that will obstruct the sliding.

Other options include, if you don't anticipate ever opening the window until spring:
* sealing the entire window as above (I do this anyway, a 19th century house is just leaky)
* using a foam spray (non-expanding is usually best, I use DAP) to spray into various gaps (cleans up with water really easily, but you still have to clean up)
* caulk (cleanup not so easy, but it's easier to apply neatly)
* duct tape (can look ugly, leave adhesive residue)
* painter's tape (like the cross-country blue car) -- cleanup easy, I have no idea how well it sticks when really really cold