good call, sinker- GM determines desired outcome, picks a number that will cause that outcome, then storytells it to fit (ie, if the number is low, say the conductor was slamming the brakes on, etc) -but a baseline still makes sense for the more type-A GMs among us.
Typically, I'd do something like figure out how much damage a fall at the subway's speed was, and use that as a benchmark for comparison, but last time I tried that, I discovered that fall damage in Dresden is a bit... overdone.
Ballpark- enough direct stress to take out anything mortal in one hit, but to also be survivable by a mortal with 'hero' status (ie, consequences, if he's willing to blow an extreme or more on it)... 7-9 sounds fair, as suggested above.
Are we thinking about this right though? It's not really a weapon being used by someone- it's a basic attack enhanced by environmental factors.
Normally that would be the purview of tapping an aspect (like <High-traffic Subway line>) to improve damage, but some environmental stuff goes way beyond an extra 2 damage...
Like... grappling someone does 1 or 2 (depending on stunts)... throwing them into a wall does 3 or 4, maybe 5 (depending on stunts, fists, and defense roll)... throwing them into <Nearby Pool of Lava> probably ought to do more than throwing them into a <Nearby Stack of Crates>.
Most other games I play have some sort of combined team effect that's far greater than the sum of it's parts... dresden... sorta does, but not to the level I'm used to.
As a GM, I'd say... have a player roll an attack, tap the declared and maneuvered aspect, then gain a further +2 for the pun (minimum)... you're looking at base damage +6 already there.
If, like me, you think some aspects should be more significant, let those aspects stack (throwing someone into Lava is tapping 3 aspects, not 1... ie, <Nearby Pool of Lava> <Lava is Hot> and <No, really. Ask a Geologist. Hot.>).