Heh. That's less "hard way" and more "btw, I decided to fill minecarts while conquering magma sea" way.
As for carving tracks into SMR - you can't smooth it, but if you dig a ramp into SMR it'll be replaced by layer material, which can be carved/smoothed.
Also, no, you can't smooth or carve Magma/Lava flow either, though the old "drop a bucket of water, then designate Construction in the step magma disappears" still works if you want to, I dunno, construct your minecart repeater in the magma sea for some reason.
Though that is little beyond the scope of the OP. For the OP, one option not mentioned so far, that doesn't break up the beautiful brook surface: You could also divert brook's waterflow from below, by using the "dig through bridges" feature. Diagram from the side:
⥳~~~
╬R▲▒
▒▲▒▒
First dig access tunnels to green ramp and stone map edge and construct retracting bridge R over green ramp, then smooth the map edge, then carve it into fortification(╬) so that you may have a drain. Then wall off the access tunnel to the level right below bridge, and then designate the red ramp for digging - dwarf will stand on green ramp while digging this. (You'll likely need to route the water to lower z-level so it can flow into map edge fortification, unless you're on a mountain, as you can't smooth soil.)
(Can't do four ramps at once without bridge due water flow pushing miners away.)
Afterwards, the water will just fall into hole and be unable to reach anything past the ramps.
PS: Also, on dipper, I'd add a downward ramp before/after wall grate to block Magma men from destroying it.