Been working on some minecart + water science.
Setup #1: Simple flat track going down ramp through a brook (4 tiles wide), up ramp to the other side, where it should dump water into a pit. No rollers. Orders are to push.
Result: Dry run test (river bed empty) resulted in the cart just barely getting up the hill on the other side. More power needed.
Setup #2: Kept same design, but put rollers in the bottom of the brook. Dry run caused the cart to succeed in getting to its destination under its own power. However, with water, it filled up and made to the ramp out of the water, but couldn't get up the hill. Instead, it just kept going partway up, then sliding back down, before the roller next to it tried to push it up again. Inspecting the cart did reveal that it was full of water, but that it weighed 479 Urists (459 of which was the water).
Setup #3: Built a 7z level tall super ramp to try to give it enough momentum to get over the other side. A dry test launched the cart airborne, overshooting the stop by at least 20 tiles before colliding with a mountain. However, the results were not better with the water. Even with rollers in the riverbed, it still couldn't make it over.
Setup #4: Kept the tower, but reduced the tiles underwater to just one track ramp. With the water, it zipped through the ramp and went airborne, but did not collect any water. (It did get a water coating, however).
Setup #5: Expanded the underwater region of the track to two track ramps adjacent to each other. However, this was still too much for it to handle, resulting it stalling at the bottom of the track.
Conclusions: The weight of the water is what kills momentum when sending a cart through it. It may be difficult but possible to get the balance just right where you can send a cart to collect water and not have it stall immediately (someone earlier in the thread was skipping minecarts, so it seems possible).
Also, minecarts that are full of water are useless, as far as I can tell.
Guiding the minecarts (I think) did get them to the end, where they did dump out their contents - however it was only 2/7 worth of it. I'm not sure minecart towers will replace pump stacks anytime soon.