I just got another awesome idea how to haul magma in minecarts without a track:
Magma Minecarts inside Wheelbarrows!
The whole design can even be completely automated!
Without the need to assign and unassign minecarts to different tracks at a time.
Plan - near the surface (side view):
Oddddd=====S==<<Zccccc
U
Near the magma (side view):
Obbbbb== ==<<Xaaaaa
\7777/
Explanation:
\7777/ - magma reservoir, with tracks in it and rollers to bring minecart up the ramp
U - here we want magma
aabbccdd - stockpiles accepting minecarts
===<< - track and rollers
S - track stop, set to lowest friction (so it doesn't stop the minecart), set to dump the contents perpendicular to the track into the U
XZ - track stops set to dump their contents to the left
O - wall to stop minecarts
The process goes like that:
1. Empty minecarts are put into stockpile aaaaa
2. There is a hauling route with one stop on X, with assigned vehicle, set to take furniture->minecarts from stockpile aaaaa.
3. Empty minecarts are put into the minecart on track stop X, the track stop dumps them to the left, placing them on the rollers.
4. Rollers move the empty minecart into the magma reservoir, they get filled with magma, then the roller on ramp moves them up, they follow the track, then go out of it and stop at the wall, effectively the minecart with magma is being placed stockpile bbbbb.
5. Stockpile ccccc is set to take from stockpile bbbbb and has assigned 3 wheelbarrows. Dwarves savely transport the minecarts with magma inside wheelbarrows into the surface into stockpile ccccc.
6. There is a hauling route with one stop on Z, with assigned vehicle, set to take furniture->minecarts from stockpile ccccc.
7. Magma minecarts are put into the minecart on track stop Z, the track stop dumps them to the left, placing them on the rollers. (same as in 3.)
8. Rollers move the magma minecart along the track, they pass through the track stop S and dump the magma in the destination point U, then they follow the track, go out of it and stop at the wall, effectively the emptied minecart is being placed stockpile ddddd.
9. Stockpile aaaaa is set to take from stockpile ddddd. Dwarves haul the empty minecarts back underground near the magma into stockpile aaaaa.
Then we go back to start and the whole think repeats.
I did a quick test and dwarves happily move minecarts inside wheelbarrows between minecart stockpiles.