I use a minecart transport system without dwarves between level 3 and 145, using impulse ramps. The down direction is mostly very simple: just a vertical drop with a ramp at the bottom. I then have it slam into a wall and fall one additional level onto another ramp in order to shed the huge velocity built up, but I don't know if that's needed. However, I also have to dodge the second cavern a bit. Upwards is completely impulse ramp driven. Currently I just send stuff down, but the system is set up so I can change it to send stuff up on the return trip. I did run into some strange issues with the impulse ramps: in 3 places they just didn't work, despite looking as it it used the same design as everywhere else. Eventually I ended up changing the route a bit, and then the ramps there worked. On the top: Route Stop with orders to push North when full. At the bottom it runs into a track stop that dumps the contents. On top of that is another route stop with order to guide south always (2 tiles into a wall), and there is another route stop with current orders to push east always. At the top I have a Track Stop with a dump south order, and overlaying that a push west route stop order into a track stop at the same location as my start route stop.
The design is usable even before you get the "up" route debugged. When hitting the bottom you don't send it further. The dorfs will then generate an order to carry the minecart back to the top.
I don't know really know how it would work with multiple minecarts. Probably possible, but requiring a fair bit of work to set it up. It doesn't hurt to add restriction zones in dangerous places, but the system has eliminated 3 cats so far, and got close to take out a human spy (a.k.a. "diplomat") and his muscle (I had to stop the operation until the morons got out of the tunnel). On that vein, remove or forbid everything in the track tunnels, as you'll probably find yourself looking at mangled corpses of scavengers otherwise. Also, separate the tracks from any foot traffic (only access through the ends) to avoid accidents with "shortcuts". A parallel staircase can help make sure that path is preferrable to the serpentine "up" track.